How do universalists avoid tempation?

I'm not a universalist, but I don't think that matters because the answer is really the same for all Christians. What you're missing is any sort of relational motivation.

When you know your parents love you unconditionally, do you take their love for granted? Do you stop caring if you upset them? If you're married, do you want to treat your spouse as though your marriage covenant with them does away with any future need for love and affection? No. Instead, the deeper the relationship, the more we want to love, cherish, and care for those in our families. We don't have a transactional attitude of trading good acts with each other. And we don't live in constant fear of being abandoned, because we trust each other.

Well these human relationships are models for the ultimate relationships we have with God. God is our ultimate Father who loves unconditionally and forgives liberally. Christ is the husband of the Church, giving his life to save and redeem us. We don't need the fear of hell to motivate us to avoid temptations and sin, because we have the positive motivations that come from our desire to please our Father, to live in harmony with Christ.

That the pain I experience when I thrust my hand into a flame may serve a beneficial purpose--because it enables me to avoid an even greater injury in the future--hardly entails that I have a good reason to thrust my hand into the flame again and again. — Thomas Talbott (The Essential Role of Free Will in Universal Reconciliation)

Talbott is known for his propagation of Trinitarian Universalism. As many Universalists, he defends that sinning brings misery and unhappiness to their lives, and that is the reason they won't keep sinning and will avoid temptation. According to Universalism, this misery brought by sin can serve a redemptive purpose because it provides a compelling motive to repent.

There are two forms of universalism - (1) Christian Universalists who believe Jesus is the only way to God and that all people will eventually accept Christ at some point throughout eternity, (2) Universalists who believe that all roads lead to God or some form of ultimate reality. I cannot speak for (2) and while I do not belong to (1), I can understand the Christian Universalists position on sin.

For any Christian, there are not only negative reasons not to sin (judgment, confession), but also positive reasons. Positive reasons include:

1. Righteous living results in Shalom - if we love God and others with our actions, we make the world a better place for everyone. Sin is bad because it breaks us and others.

2. Obedience to God results in peace and joy that no circumstances can take away. For those with God's Spirit in their hearts, obedience to God brings joy because we obey out of love for God.

3. Obedience results in peace in our lives and in our hearts. It is the peace of knowing that we are at peace with God and others to the best of our ability.

True obedience to God is motivated by love for God; not chiefly by fear. So a Christian Universalist has every reason to obey God out of love.

Moreover, the Christian Universalists believes that one day all people will obey God - so why not start now?

In fact, George Macdonald, a well known Christian Universalist, used the following rationale to show that God's goal is not the punishment of sin but to bring every person to a place where they loathe and overcome the sin in their lives. This quote shows that to the Christian Universalist sin itself is the enemy to be defeated to heal humanity.

“Punishment, I repeat, is not the thing required of God, but the absolute destruction of sin. What better is the world, what better is the sinner, what better is God, what better is the truth, that the sinner should suffer—continue suffering to all eternity? Would there be less sin in the universe? Would there be any making-up for sin? Would it show God justified in doing what he knew would bring sin into the world, justified in making creatures who he knew would sin? What setting-right would come of the sinner’s suffering? If justice demand it, if suffering be the equivalent for sin, then the sinner must suffer, then God is bound to exact his suffering, and not pardon; and so the making of man was a tyrannical deed, a creative cruelty. But grant that the sinner has deserved to suffer, no amount of suffering is any atonement for his sin. To suffer to all eternity could not make up for one unjust word. Does that mean, then, that for an unjust word I deserve to suffer to all eternity? The unjust word is an eternally evil thing; nothing but God in my heart can cleanse me from the evil that uttered it; but does it follow that I saw the evil of what I did so perfectly, that eternal punishment for it would be just? Sorrow and confession and self-abasing love will make up for the evil word; suffering will not. For evil in the abstract, nothing can be done. It is eternally evil. But I may be saved from it by learning to loathe it, to hate it, to shrink from it with an eternal avoidance. The only vengeance worth having on sin is to make the sinner himself its executioner.”