You have this problem? D.A. Carson calls this mental drift, and I've found his comments on this issue very helpful. I thought you might benefit from them as well. They come from his book A Call to Spiritual Reformation (i.e. best book on prayer I've read).
Hope this helps:
Anyone who has been on the Christian way for a while knows there are times when our private prayers run something like this: “Dear Lord, I thank you for the opportunity of coming into your presence by the merits of Jesus. It is a wonderful blessing to call you Father….I wonder where I left my car keys? [No, no! Back to business.] Heavenly Father, I began by asking that you will watch over my family-not just in the physical sphere, but in the moral and spiritual dimensions of our lives….Boy, last Sunday’s sermon was sure bad. I wonder if I’ll get that report written on time? [No, no!] Father, give real fruitfulness to that missionary couple we support, whatever their name is….Oh, my! I had almost forgotten I promised to fix my son’s bike today….” Or am I the only Christian who has ever had problems with mental drift? But you can do many things to stamp out daydreaming, to stifle reveries. One of the most useful things is to vocalize your prayers. This does not mean they have to be so loud that they become a distraction to others, or worse, a kind of pious showing off. It simply means you articulate your prayers, moving your lips perhaps; the energy devoted to expressing your thoughts in words and sentences will order and discipline your mind, and help deter meandering. Another thing you can do is pray over the Scriptures. Christians just setting out on the path of prayer sometimes pray for everything they can think of, glance at their watches, and discover they have been at if for all of three or four minutes. This experience sometimes generates feelings of defeat, discouragement, even despair. A great way to begin to overcome this problem is to pray through various biblical passages. In other words, it is entirely appropriate to tie your praying to your Bible reading. The reading schemes you may adopt are legion. Some Christians read a chapter a day. Others advocate three chapters a day, with five on Sunday: this will get you through the Bible in a year. I am currently following a pattern set out by Robert Murray M’Cheyne in the last century: it will take me through the Psalms and the New Testament twice during this calendar year, and the rest of the Old Testament once. Whatever the reading scheme, it is essential to read the passage slowly and thoughtfully so as to retrieve at least some of its meaning and bearing on your life. Those truths and entailments can be the basis of a great deal of reflective praying. A slight variation of this plan is to adopt as models several biblical prayers. Read them carefully, think through what they are saying, and pray analogous prayers for yourself, your family, your church, and for many others beyond your immediate circle. Similarly, praying through the worship sections of the better hymnals can prove immensely edifying and will certainly help you to focus your mind and heart in one direction for a while. Some pastors pace as they pray. One senior saint I know has long made it his practice to pray through the Lord’s Prayer, thinking through the implications of each petition as he goes, and organizing his prayers around those implications. Many others make prayer lists of various sorts, a practice that will be discussed in more detail later. This may be part of the discipline of what has come to be called “journaling.” At many periods in the history of the church, spiritually mature and disciplined Christians have kept what might be called spiritual journals. What such journals contain varies enormously. The Puritans often used them to record their experiences with God, their thoughts and prayers, Their triumphs and failures. Bill Hybels, the senior pastor of Willow Creek Community Church, takes a page to record what he did and thought the day before, and then to write out some prayers for the day ahead of him. At least one seminary now requires that their students keep such a journal throughout their years of study.
The real value of journaling, I think, is several-fold: (a) It enforces a change of pace, a slowing down. It ensures time for prayer. If you are writing your prayers, you are not daydreaming. (b) It fosters self-examination. It is an old truism that only the examined life is worth living. If you do not take time to examine your own heart, mind, and conscience from time to time, in the light of God’s Word, and deal with what you find, you will become encrusted with the barnacles of destructive self-righteousness. (c) It ensures quiet articulation both of your spiritual direction and of your prayers, and this in turn fosters self-examination and therefore growth. Thus, journaling impedes mental drift. But this is only one of many spiritual disciplines. The danger in this one, as in all of them, is that the person who is formally conforming to such a regime may delude himself or herself into thinking that the discipline is an end in itself, or ensures one of an exalted place in the heavenlies. That is why I rather oppose the imposition of such a discipline on a body of seminary students (however much I might encourage journaling): true spirituality can never be coerced.
D.A. Carson, A Call to Spiritual Reformation: Grand Rapids, Baker Book House, 1992, p. 20-22.