It is not so true that "prayer changes things" as that prayer changes me and I change things. God has so constituted things that prayer on the basis of Redemption alters the way in which a man looks at things. Prayer is not a question of altering things externally, but of working wonders in a man's disposition.
Get into the habit of dealing with God about everything. Unless in the first waking moment of the day you learn to fling the door wide back and let God in, you will work on a wrong level all day; but swing the door wide open and pray to your Father in secret, and every public thing will be stamped with the presence of God.
Intercessory prayer for one who is sinning prevails. God says so! The will of the man prayed for does not come into question at all, he is connected with God by prayer, and prayer on the basis of the Redemption sets the connection working and God gives life.
Prayer is often a temptation to bank on a miracle of God instead of on a moral issue, i.e., it is much easier to ask God to do my work than it is to do it myself. Until we are disciplined properly, we will always be inclined to bank on God's miracles and refuse to do the moral thing ourselves. It is our job, and it will never be done unless we do it.
If God sees that my spiritual life will be furthered by giving the things for which I ask, then He will give them, but that is not the end of prayer. The end of prayer is that I come to know God Himself.
Human nature, if it healthy, demands excitement; and if it does not obtain its thrilling excitement in the right way, it will seek it in the wrong. God never makes bloodless stoics; He makes no passionless saints.