If it's starting to do it, there is a good chance that you have the belt too tight. Overtight belts are fairly common. At the tightest, 1/2" deflection, and it should be a generous half inch. I keep mine fairly loose--probally closer to an inch deflection--because looser belts give a nano-amount of additional hp, and free nano-amounts of hp are good things. It also extends belt life, and provides more elastcity (sp) helping eliminating the latent chance of snapping the belt during hard acceleration.

A new belt is loose enough so that after a short while, it starts to slip after it stretches. Afterward, I tighten the belt just enough so that it doesn't slip.