Another good thing that I found to work on trigger points is a spikey ball. The set I have came with a larger, and softer ball, that actually looks more like a peanut. That one's okay, but the one I really like is a smaller 2 inch ball that is really hard. Those little spikes on the ball really dig into your muscles. It's almost like a meat tenderizer.
Not only do the little spikes dig into the muscles well, but they also help hold the ball in place. That's one problem I've had with with other kinds of balls. If you get on a tight spot, the ball squirts out of there. This one doesn't do that. It makes it much easier to use.
Here's what they look like.
http://www.youtube.com/watch?v=957tflebvxM