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

I will give a Lutheran response.

There are various points of view among Lutheran theologians on being filled or “baptized in the Holy Spirit." The dominant view is that the function of being “baptized in the Holy Spirit” is an ongoing reality of the Christian life that first occurs at conversion.

Still other Lutheran theologians have highlighted the privileged position that one has in the rite of initiation to be “baptized in the Holy Spirit" in terms of being empowered for service.

In this view, the gift of the Holy Spirit is distinquished but not seperated from initiation. In addition to not being dogmatic cessationists, Lutherans have also made a distinction between the charismatic activity of the Spirit and the regenerative activity of the Spirit. For example, the 19th century theologian, Francis Pieper, writes:

Lutheran theologians accordingly distinguish between Baptismus Sanquinis, or martyrdom (Matthew 20), Baptismus Flaminis, or the outpouring of the special gifts of the Holy Spirit (Acts 1:5), and Baptismus Fluminus, or baptism of water which is the sacrament for the remission of sins.” (Pieper, Christian Dogmatics, Volume, III, p. 289)

Another Lutheran theologian, P.E. Kretzman, in an article back in 1924 for the Concordia Publishing House called “The Baptism or Gift of the Holy Ghost” also argues:

The special baptism, or gift, of the Holy Ghost in the apostolic period, distinct from regeneration or conversion, consisted in this, that He endowed the believers of that day with extraordinary miraculous gifts, such as speaking in tongues, prophesying, performing miracles, etc.

After the mentioning of the Cornelius event in Acts 10, Kretzman continues:

We find furthermore that in most cases the special act of laying on of hands accompanied the imparting of the Spirit, and that a distinct filling of the Holy Ghost, in addition to that taking place in regeneration is spoken of.

In his 1921 commentary on Acts 8, Kretzman writes about the prayers of the apostles for the Samaritans to receive the Holy Spirit:

The Samaritans had been baptized, and therefore they were in full possession of the pardon of God, as well as of the Spirit which sanctifies, Mark 16,16; Acts 2,38. But now they were equipped with extraordinary gifts, with the power to perform miracles, to speak with strange tongues, to prophesy, and to give other peculiar evidences of the Spirit’s omnipotence and divine majesty. These extraordinary manifestations had not yet been imparted to these believers, although all the spiritual gifts were theirs by and through Baptism. But now these powers were transmitted to them by the laying on of hands, for it was a part of the Lord’s plan in the early Church to use miracles and signs to confirm the preaching of the Gospel. (Kretzman, Popular Commentary: Book of Acts)

What Kretzman writes is in line with what Martin Luther taught. For example, in commenting on Acts 10, Luther writes:

Cornelius had the Holy Spirit before Peter came to him, although he didn’t know it. Those in the book of Acts who said, ‘We don’t know the Holy Spirit,’ also had the Spirit, just as the patriarchs in the Old Testament had Christ, although they didn’t know him. They clung to the Word, and through it they received the Holy Spirit. Later in the book of Acts he was manifested to them outwardly.

The Augsburg Confession is very bold in its insistence:

As can be seen, there is nothing that varies from the Scriptures, or from the Church universal, or from the Church of Rome, as known from its writers. (Conclusion of first half of the AC)

In doctrine and ceremonies we have received nothing contrary to Scriptures or the Church universal. (Conclusion of the second half of the Augsburg Confession).

Here are some quotes from the early church in regards to the gift of the Spirit.

Cyril of Jerusalem, writing around 340 A.D., states:

(Catechumens) who are about to be baptized even now in the Holy Spirit...Let each one prepare oneself to receive the heavenly gift...according to your capacity...God grant that you may be worthy of the charism of prophecy...My final words, beloved brethren, in this instruction, will be words of exhortation, urging all of you to prepare your souls for the reception of the heavenly charisms...” (CL 16:6; 17:37; 18:32)

Hilary of Poitiers, writing around 356 A.D., also links the rite of initiation with an experience of the Holy Spirit and the charisms:

... After the water-bath, the Holy Spirit rushes upon us from the gate of heaven, that we might bathe in the anointing of the holy glory, and that we might become sons of God through adoption spoken by the voice of the Father...(believers are given a share in)...diverse gifts and charisms.’” (On Matthew, 2:6; 15:10)

The early 3rd century church father, Tertullian, included a passage indicating that prayer and charismatic gifts accompanied the “new birth” of one who became a Christian. He exhorts:

Therefore, blessed ones, whom the grace of God awaits, when you ascend from that most sacred font of your new birth, and spread your hands for the first time in the house of your mother, together with your brethren, ask from the Father, ask from the Lord, that His own riches of grace and distributions of charisms (peculia gratiae distributiones charismatum subiacere) may be supplied to you. ‘Ask,’ he says, ‘and you shall receive’. Well, you have asked, and have received; you have knocked, and it has been opened to you. (Tertullian, On Baptism, 20).

According to Tertullian, the receiving of the Holy Spirit is among the essential fruits of baptism, along with the remission of sins, deliverance from death, and regeneration (Against Marcion 1,28). Several other passages also suggest that one receives the Spirit in the sacrament of baptism (On Modesty IX, 9; and On the Soul I,4).

The authors of the Lutheran Confessions also write about the importance of a proper disposition in receiving sacramental grace:

The Holy Spirit wills to work such repentance and faith in us through the Word and the sacraments. And in order that we may see it through and abide and persevere in that we should implore God to give us his grace, of which he has assured us in holy Baptism, and not doubt that according to his promise he will give it to us. We have his word, ‘What father among you, if he son asks for a fish, will instead of a fish give him as serpent; or if he asks for an egg, will give him a scorpion? If you then, who are evil, know how to give good gifts to your children how much more will the heavenly Father give the Holy Spirit to those who ask him?’ (Luke 11:11-13) (Formula of Concord, SD, XI, 71-72)

Lutheran theologians make a distinction between active faith (i.e.fides heroica) filled Christians and “holy” people who have fallen into open sin. The Lutheran Confessions state:

It is therefore necessary to know and to teach that when holy people, aside from the fact that they still possess and feel original sin and daily repent and strive against it, fall into open sin (as David fell into adultery, murder, and blasphemy), faith and the Spirit have departed from them. This is so because the Holy Spirit does not permit sin to rule and gain the upper hand in such a way that sin is committed, but the Holy Spirit represses and restrains it so that it does not do what it wishes. If the sin does what it wishes, the Holy Spirit and faith are not present . . . (Smalcald Articles, III, III, 43.)

Of course, how one defines “ordinary” believers is subject to debate. Should we make distinctions based upon the manifestation of certain charisms and/or physiological experiences?

One of the problems of a Pentecostal distinction between Spirit-baptized Christians and other Christians is that it can easily foster a divisive notion that the former constitute a spiritually elite class of Christians.

While the sacrament of initiation conveys the Holy Spirit, orthodox Lutherans would agree with how the late Leon Joseph Cardinal Suenens taught concerning a sacramental understanding of the continuing work of the Holy Spirit:

...we must assume the fundamental fact that a baptized Christian has already received the fullness of the Holy Spirit. The Spirit is not still on his way, he is already radically present from the very beginning of the Christian life,...During the liturgy of Advent we pray, again and again, to the Father to send his Son, as though the son had not yet become incarnate...We must interpret charismatic language the same way...The Spirit is sent whenever he is perceived. (Suenens, A New Pentecost, pages 87-88)

While Lutherans would agree with Suenens' statement, they would also want to point out the the Means of Grace in general - i.e. the hearing of faith (Galatians 3:1-5), should not be depreciated in the midst of highlighting the rite of initiation. This is in line with early evangelical catholic theology, in that St. Thomas Aquinas once asked the question whether we can speak of a sending of the Spirit to a person in whom he is already indwelling, and if so, how this is to be understood. Aquinas answers as follows:

There is an invisible sending also with respect to an advance in virtue or an increase of grace . . . . Such an invisible sending is especially to be seen in that kind of increase of grace whereby a person moves forward into some new act or some new state of grace: as, for instance, when a person moves forward into the grace of working miracles, or of prophecy, or out of the burning love of God offers his life as a martyr, or renounces all his possessions, or undertakes some other such arduous thing. [Summa Theologiae I, q. 43, a. 6, ad 2um]

So theologically, Lutherans as evangelic catholics, would argue that Christians are continually "filled with the Holy Spirit" through the hearing of faith (i.e. Word & Sacrament). However, experientially, Christians can manifest the Holy Spirit in different ways at different times in their lives according the disposition of faith.