Perhaps the most humbling aspect of the attempt to engage in prayer is the difficulty of controlling one’s thoughts. Yet the mercy of God is revealed in the divine patience toward such failures of attention (Explanation Of The Psalms. Ps. 85.7). In fact, the very difficulty, even impossibility, of controlling thoughts Augustine considered to be evidence of the necessity of grace (On The Gift Of Perseverance. 8.19–20).

Rebecca H. Weaver, “Prayer,” ed. Allan D. Fitzgerald, Augustine through the Ages: An Encyclopedia (Grand Rapids, MI; Cambridge, U.K.: William B. Eerdmans Publishing Company, 1999), page 673.

My notes:
The above quote (bold emphasis added by me) concerns the mental focus we need to learn so we can pray better. Two of the additional attributes of disciple that St. Augustine teaches are:

1) Pray at set times. Not at just one time of the day, but multiple times through the day.
2) Use short prayers. Two good examples are:

a) The Jesus prayer: “Lord Jesus Christ, Son of God, have mercy on me, a sinner.”
b) The Doxology – “Glory be to the Father, and to the Son, and to the Holy Spirit,
as it was in the beginning, is now, and ever shall be, world without end.”

So ask Jesus for the grace necessary to focus better when you pray (pray for help to be better at prayer).