The first means of avoiding Purgatory is to remove the cause which sends us there, which is sin.
 
It may not be easy to refrain from all sin, even the smaller sins, but every ordinary Christian can, by the frequent use of the Sacraments, easily abstain from mortal sin.

Secondly, we can all avoid deliberate and grave venial sin. It is an awful thing to offend the good God deliberately. Deliberation intensifies enormously the malice of sin and offends God much more than faults of weakness, or sins committed when we are of our guard.

Lastly, we must use our best efforts to break off bad habits. Habits, like deliberation, add seriously to the malice of sin. A deliberate lie is much worse than a hasty lie of excuse, and a lie resulting from the deep-rooted habit of lying is much worse than a casual lie.

A certain lady had the habit, when she was younger, of constantly speaking ill of her neighbors. After listening to a sermon on the subject of lying, she made a strong resolution never to do so again, and kept it. That simple, strong resolution changed the whole trend of her life and saved her from thousands of sins, and most surely from a long and painful Purgatory.

If a Christian avoids, as he easily can, these three classes of sin;
mortal sins, deliberate and grave venial sins, and habits of sin, it will be relatively easy for him to atone for faults of weakness.

We would be well advised to pronounce with special emphasis and fervor, every time we say the Our Father, the words: "Forgive us our trespasses as we forgive those who trespass against us." These are the very words of God Himself and repeated frequently and fervently will certainly obtain for us pardon of our sins.
 

(Source:    How to Avoid Purgatory by Fr. Paul O'Sullivan)