I used to do this as part of drama class - training performers to be able to relax is a great skill! We would all fall asleep on the floor of the stage.

I think a really helpful way to do this is to have someone with a soothing voice read the instructions to you, just like Buggs said. My voice makes me squirm when I've recorded it, but I'm sure there are audio recordings for this. My drama teacher had a really relaxing sort of way of reciting the whole script for relaxation. It would prevent you from getting distracted, or if you're all wound up in your thoughts, then it would help distract you from that sort of perseveration.