Also make sure you have your clutch pretty close to straight when figuring belt length. The last one I built I didn't do that and ended up with an inch too long belt. It worked fine for a few laps but once the belt stretched a little the clutch was out of pull (spring not keeping belt tight), and my belt slipped really bad. I revised the linkage so I could run an inch shorter belt and the slipping issue is gone now. Look at George's 07 build, he has some good pictures of a correct belt clutch setup and you will see what I mean by having the clutch close to straight. It's also a good idea to incorporate as much pull in the clutch (release and spring pull) as you can. If not it'll either slip or you won't be able to get it to release.
Hope this helps you out, if it don't then let us all know what did help you get it right.
