I've always loved Psalm 121. It's a simple reminder that God is sovereign and He loves me.
One of the best applications of that Psalm was a message I listened to from Menlo Park Presbyterian Church. It was a part of a great series they did on "Christian Atheism"- ways that our belief in God doesn't show through our lives.
I began listening to this message thinking I didn't worry as much as I used to- that I really had this one under control. Then the speaker talked about trying to control worry with things like workaholism or perfectionism.
Oh. He got me there.
Definitely worth a listen.
I Believe in God, but I Still Worry a Lot by John Ortberg.