belt throwing

Having had more belts thrown in the past year than most folks will see in there life time, I'll throw in my 2 cents. After checking your pulley alignment, also check that the alternator is square to the crank pulley (part of my cars issue). My small block stroker would toss belts almost every single round of racing. Lots of times I would just remount the belt, other times it would loose it completely or chew it up. I always kept 3 spares in my trunk (no joke)....and some races I needed ALL of them. For my car, the best way to keep the belt on was:

1. Run the belt between 3 points instead of 2 (water pump, balancer, alternator). I was just running a belt from the balancer to the alternator and using an electric water pump, but this made the belt throwing worse.
2. Use the widest, stiffest best you can fit.

The belt that I use now that stays on about 95% of the time, and it is actually wider than the grove on the pulley (it sits a bit out of the groove, but its only 5 bucks from Autozone). I believe the part number is 17465. The 15465 works well too, but I have had better luck with the wider one (17465). I also found a nice thick one at NAPA that worked great, but it was close to 30 bucks.

By the way, my car might be a bit of an extreme example because I race it a LOT...probably 80-90% race, the rest street.