Martin Luther had a barber named Peter Beskendorf who asked his world-famous customer and doctor of theology, "Dr. Luther, how do you pray?"
Martin Luther answered with a letter of forty printed pages which was published in the spring of 1535, under the title A Simple Way to Pray, for a Good Friend.
It is a wonderful letter which not only tells us about Martin Luther's personal spiritual life but is filled with contextual and empathetic counseling.
Dear Master Peter,
I give you the best I have. I tell you how I pray myself. May our Lord God grant you and everyone to do it better.
A good clever barber must have his thoughts, mind and eyes concentrated upon the razor and the beard and not forget where he is in his stroke and shave. If he keeps talking or looking around or thinking of something else, he is likely to cut a man's mouth or nose - or even his throat. So anything that is to be done well ought to occupy the whole man with all his faculties and members. As the saying goes: he who thinks of many things thinks of nothing and accomplishes no good. How much more must prayer possess the heart exclusively and completely if it is to be a good prayer.
Walter Trobisch reprinted Luther’s classic, booklet on meditation with insightful comments in 1975 as Martin Luther's Quiet Time.