Crunches, and the Swiss Ball. The balls are cheap, around $20 at most sports stores, and they often have exercise diagrams included. Mine even had a short video. I am amazed at how I can feel the muscles in the abdomen tighten, and how good my shoulders and back feel after a few minutes on the ball. If you're like me and don't like to spend alot of money or time "exercising," this is the way to go. I'd rather ride or run for a half-hour than do some exercise routine!