I found a book a while back that I had heard of, but never seen. It's called The Practice of the Presence of God by Brother Lawrence. He became a Carmelite monk in 1666, his place of service was the kitchen. The book is a record of conversations with him and letters from him to friends. I hadn't intended to include it as part of the "Shadows" posts, because I discovered it after I had written that study. I found this in the Eighth Letter which discussed the problem of "wandering thoughts in prayer" and decided it was good not to mention.
"One way to recollect the mind easily in the time of prayer, and preserve it more in tranquillity, is not to let it wander too far at other times. You should keep it strictly in the presence of God; and being accustomed to think of Him often, you will find it easy to keep your mind calm in the time of prayer, or at least to recall it from its wanderings."
I think this is the most amazing advice because it is so simple. If your mind wanders when you're praying, try concentrating on God even when you're NOT praying. You will get into the practice of being with Him; you will be aware of His presence. If you think of it in human terms, that's we way we operate. I don't have to keep reminding myself that my husband is in the room with me. I'm always aware of him because I value his presence. I want to be with him. It works the same way with God. Our mind will be on what we value. We are on His mind because He values us. How much do we value being with Him?