According to Catholicism, what are concrete and practical ways by which Christians can be filled (and stay filled) with the Holy Spirit?

You are asking about the so-called indwelling (inhabitatio) of the Holy Ghost.

What does it mean to be "filled" with the Holy Spirit?

"Indwelling" is, according to Catholic Dictionary by Fr. Hardon, S.J.:

Presence of the Holy Spirit in a person who is in the state of grace. He is present not only by means of the created gifts of grace, which he dispenses, but by his uncreated divine nature. This personal indwelling does not produce a substantial but only an accidental union with the souls of the just. As the indwelling of the Holy Spirit is an operation of God outside himself and as all activity of God outside the Trinity is common to the three persons, the indwelling of the Holy Spirit implies the indwelling of the three divine persons. This indwelling as a manifestation of the love of God, the personal love of the Father and the Son, is appropriated to the Holy Spirit. St. Paul speaks of the third person: "Know you not that you are the temples of God and that the Spirit of God dwells in you" (I Corinthians 3:16). But he also says: "You are the temple of the living God" (I Corinthians 6:16), and Christ declares: "If any one loves me, he will keep my word. And my father will love him; and we will come to him and will make our abode with him" (John 14:23).

The immediate effect of the divine indwelling is sanctifying grace, which is the created result of the uncreated grace of God's presence. Its effect on the person is an experience that spiritual writers compare to a foretaste of the beatific vision; the mind is able to understand something of the mystery of God and the will is enamored of his goodness beyond anything possible by the light of reason or the natural affective powers of humans.

Pope Leo XIII, encyclical Divinun illud munus on the Holy Ghost:
^{(quoted in Garrigou-Lagrange, O.P., Three Ages of the Interior Life, pt. 1, ch. 4 "The Blessed Trinity Present in Us, Uncreated Source of our Interior Life")}

9. […] God is in man, not only as in inanimate things, but because he is more fully known and loved by him, since even by nature we spontaneously love, desire, and seek after the good. Moreover, God by grace resides in the just soul as in a temple, in a most intimate and peculiar manner. From this proceeds that union of affection by which the soul adheres most closely to God, more so than the friend is united to his most loving and beloved friend, and enjoys God in all fullness and sweetness. Now this wonderful union, which is properly called “indwelling,” differing only in degree or state from that with which God beatifies the saints in heaven, although it is most certainly produced by the presence of the whole Blessed Trinity — “We will come to Him and make our abode with Him” (John 14:23) — nevertheless is attributed in a peculiar manner to the Holy Ghost.

What are concrete and practical ways by which Christians can be filled (and stay filled) with the Holy Spirit?

Staying in a state of sanctifying grace (not having committed a mortal sin since one's last confession) is the main sign of the presence of the Holy Ghost in one's soul.

Garrigou-Lagrange, O.P., Three Ages of the Interior Life, pt. 4, ch. 40 "The Spiritual Age of the Perfect, Their Union with God ", § "The Signs of the Indwelling of the Blessed Trinity in the Purified Soul", following St. Thomas Aquinas (Summa contra Gentiles lib. 4 cap. 21ff.; Summa Theologica I-II q. 112 a. 5), lists the signs of how "a man can know if he is in the state of grace":

1. "testimony of a good conscience"
2. "joy in hearing the word of God preached"
3. "relish of divine wisdom"
4. "inclination leading the soul to converse intimately with God"
5. "to rejoice in God, fully consenting to His will"
6. "the liberty of the children of God"
7. "speaks of God out of the abundance of his heart"

First, about terminologies

In Catholicism the concept of "being filled with the Holy Spirit" in connection with Charismatic's understanding of Baptism with the Holy Spirit or with Eph 5:18 filling in post-conversion / post-confirmation state, is not common until the 1960s when the Catholic Charismatic movement started to borrow the wider Charismatic concepts. When the phrase "filled with the Holy Spirit" WAS used occasionally prior to the 1960s, it usually refers to either Baptism or Confirmation.

The main term in Catholicism is instead Indwelling, defined as:

Presence of the Holy Spirit in a person who is in the state of grace. He is present not only by means of the created gifts of grace, which he dispenses, but by his uncreated divine nature. This personal indwelling does not produce a substantial but only an accidental union with the souls of the just. ...

The immediate effect of the divine indwelling is sanctifying grace, which is the created result of the uncreated grace of God's presence. Its effect on the person is an experience that spiritual writers compare to a foretaste of the beatific vision; the mind is able to understand something of the mystery of God and the will is enamored of his goodness beyond anything possible by the light of reason or the natural affective powers of humans.
_{(Source: Catholic Dictionary - Indwelling)}

For this answer I try to find the best approximation with how the term is used within the wider Charismatic movement, which I think is similar to the effects of the Catholic Sacrament of Confirmation, although we need to remember that this sacrament does not stand alone but is usually given in conjunction with the other two sacraments of initiation: Baptism and The Eucharist.

What are concrete and practical ways by which Christians can be filled (and stay filled) with the Holy Spirit?

In Catholicism, it's through

• the one time Sacrament of Baptism where we receive the Holy Spirit's indwelling and the seven gifts of the Holy Spirit,
• the one time (CCC 1304) Sacrament of Confirmation which seals and strengthens the gifts within us, and
• the maintenance of the indwelling of the Holy Spirit and His gifts by keeping ourselves in the state of grace (through not committing a mortal sin).

At Baptism, we receive seven special gifts from the Holy Spirit. These gifts are freely given to us to help us live as followers of Jesus and to build up the Body of Christ, the Church. The Gifts of the Holy Spirit are sealed and strengthened within us at Confirmation.
_{(source: RCL Benziger: Confirmation: Gifts and Fruits of the Holy Spirit)}

What does it mean to be "filled" with the Holy Spirit?

If in Baptism it is the Holy Spirit who immerses us in Christ, then in Confirmation it is Christ who fills us with his Spirit, consecrating us as his witnesses, participants in the same principle of life and of mission, according to the design of the heavenly Father.
_{(source: Pope Francis General Audience "Filled with the Holy Spirit", 25 May 2018)}

In short [Confirmation] is the full outpouring of the Holy Spirit as once granted to the apostles on the day of Pentecost. Confirmation brings Catholics a deepening of baptismal grace and unites us more firmly to Christ. It increases the gifts of the Holy Spirit and leaves an indelible mark on the soul just like baptism.
_{(source: Catholic Confirmation Explained)}

The seven gifts of the Holy Spirit received at Baptism and strengthened at Confirmation are

1. Wisdom,
2. Understanding,
3. Knowledge,
4. Fortitude or Courage,
5. Counsel,
6. Piety or Love, and
7. Fear of the Lord.

What is the requirement for the recipient to receive the Sacrament of Confirmation?

CCC 1306: Every baptized person not yet confirmed can and should receive the sacrament of Confirmation. ...

CCC 1310: To receive Confirmation one must be in a state of grace. One should receive the sacrament of Penance in order to be cleansed for the gift of the Holy Spirit. More intense prayer should prepare one to receive the strength and graces of the Holy Spirit with docility and readiness to act.

However, like all sacraments of the Catholic Church, the principle ex opere operato ("from the work performed") applies. This means that the efficacy of the sacrament, which is the instrumental cause of grace,

• doesn't depend on the effort of the recipient (although the person should be properly disposed, i.e. has openness to the grace given in the sacrament)
• doesn't depend on the holiness of the priest, because it is Christ himself who is the author of each sacrament
• while a proper disposition (openness) is necessary to exercise the efficacious grace in the sacraments, it is not the cause of the sufficient grace.

  Catholic Christians believe that what God offers in the sacraments is a gift, freely bestowed out of God’s own love. A person's disposition, as good as it may be, cannot merit supernatural grace or divine life, which remains a gift of God.
  _{(source: wikipedia: Ex opere operato in the Catholic Church)}

CCC 1308 also reminds us that

... the baptismal grace is a grace of free, unmerited election and does not need "ratification" to become effective. ...

further emphasizing that the efficacy is NOT through the effort of the recipient.

How can one see that we have received the Gift of the Spirit?

If we fulfil the works of the Spirit, if we speak the words instructed by the Spirit (cf. 1 Cor 2:13). Christian witness consists in doing only and all that the Spirit of Christ asks of us, giving us the strength to accomplish it.
_{(source: Pope Francis General Audience "Filled with the Holy Spirit", 25 May 2018)}

For a background explanation of the Catholic tri-partite theory of sacrament, see attachment to a related question.

The traditional view can be found a three volume German theological work by Joseph Pohle into the English language. Pohle was a Jesuit of the Molinist persuasion. The series was edited and published in 1915 by Arthur Preuss (1871-1934). This three volume dogmatics series was standard reading in American Catholic seminaries up until the time of the Second Vatican Council.

It is interesting that Arthur Preuss was the son of Eduard Preuss - a noted Lutheran theologian who joined the Roman Catholic church. Among other things, before swimming across the Tiber, Eduard Preuss was known for editing Martin Chemnitz’s Examen Concilii Tridentini and Justification of a Sinner before God.

In his editing of the German theological work by Joseph Pohle, Arthur Preuss (or Pohle?) states that "Confirmation often bestowed those extraordinary gifts (gratiae datae) known as charismata." However, Preuss (or Pohle?) also writes:

In the Apostolic Church, Confirmation often bestowed those extraordinary gifts (gratiae datae) known as charismata, e.g. speaking in divers tongues, prophesying future events, discerning good spirits from evil, etc. The existence of these gifts may be traced in the writings of the sub-Apostolic Fathers, especially St. Ignatius of Antioch, St. Polycarp, St. Justin Martyr, and St. Irenaeus. The charismata had ceased in the time of St. Chrysosotom... (Joseph Pohle & Arthur Preuss, The Sacraments, p. 315, A footnote cites 1 Corinthians 12:1)

It should be noted that the word “often” does not mean the same as “always.” Preuss (or Pohle?) argued that the charismata were not limited in distribution to the imposition of hands. For example, he writes:

The Protestant objection that the imposition of hands had for its sole purpose the conferring of certain extraordinary gifts (charismata) such as speaking with divers tongues, prophesying, etc., is refuted by the fact that those gifts were sometimes bestowed without any external rite (Acts 10:44) and that they neither invariably nor necessarily accompanied Confirmation (1 Corinthians 12:30). (Joseph Pohle & Arthur Preuss, The Sacraments, p. 295)

Since the time of the Vatican Council there has been a lot of additional research and theological reflection related to the the rites of initiation in relationship to the charismata. With the advent of the Charismatic renewal movement in the 1960’s the early Augustinian opinion on the cessation of the charismata has been reexamined, especially on the basis that the gift of prayer jubilation appears likely to be a variant species of the gift of tongues described in the New Testament.

Fr. George Montague and Fr. Killian McDonnell have written a major work tracing the biblical and historical experience of the Spirit in relationship to the sacraments of Christian initiation. Their book is: Christian Initiation and Baptism in the Holy Spirit (Collegeville: The Liturgical Press, A Michael Glazier Book).

In his analysis of patristic sources, Fr. George McDonnell confirmed that the charisms of the Holy Spirit, including the gifts of tongues and prophecy, were indeed sought for and received during the rites of Christian initiation (baptism, confirmation, Eucharist). In the research he provides evidence from witnesses around the Mediterranean seaboard that extends from the end of the second to the eighth centuries of the Christian era.

In the official 1974 "Catechism of the Catholic Church" it states:

... the Church invites us to call upon the Holy Spirit every day, especially at the beginning and the end of every important action...the traditional form of petition to the Holy Spirit is to invoke the Father through Christ our Lord to give us the Consolor Sprit... (section 2670, 2671)

And:

'No one can say 'Jesus is Lord' except by the Holy Spirit.’ Every time we begin to pray to Jesus it is the Holy Spirit who draws us on the way of prayer by his prevenient grace. Since he teaches us to pray by recalling Christ, how could we not pray to the Spirit too? That is why the Church invites us to call upon the Holy Spirit every day, especially at the beginning and the end of every important action...The traditional form of petition to the Holy Spirit is to invoke the Father through Christ our Lord to give us the Consoler Spirit. See here.

Here are some practical ways to pray to be filled with the Holy Spirit, from the Papal preacher - i.e. Fr Raniero Cantalamessa, OFM Cap. Sourced from this site:

To those who like liturgical prayer, I suggest repeating several times a day, at your choice, one of the following invocations to the Holy Spirit used in the liturgy, knowing that you are joining the countless groups of believers who have prayed them before us:

“Come, Holy Spirit, fill the hearts of your faithful and kindle in them the fire of your love”. (For those who still love to pray with the original Latin formulas: “Veni, Sancte Spiritus, reple tuorum corda fidelium et tui amoris in eis ignem accende”.) Or: “Send your Spirit, Lord, and renew the face of the earth”. Or: “Come, Creator Spirit, visit our minds, fill the hearts you have created with heavenly grace”.

To the English-speaking brothers and sisters, I suggest repeating, either alone or in the group, the words of that song we received from the Pentecostal brothers and that accompanied millions of believers when receiving the baptism in the Spirit (alternating the singular “me” with the plural “us”):

“Spirit of the living God, fall afresh on US: melt US, mold US, fill US, use US. Spirit of the living God, fall afresh on US."

In my book commenting the Veni Creator, I have also drafted an invocation to the Holy Spirit of my own. In this circumstance, I gladly share it with those who might feel inspired by it: Come, Holy Spirit! Come, strength and sweetness of God! Come, You, movement and peace! Renew our courage, Fill our solitude in the world, Create in us intimacy with God! We no longer say, like the prophet: “Come from the four winds”, As if we did not yet know from where you came; We say: Come, Spirit from the pierced side of Christ upon the cross! Come from the mouth of the Risen One!