Change the location of your ponytail often - wear it high, low, on one side, then the other. Don't wear a ponytail at all if your hair isn't really long enough to need one... otherwise you'll probably be pulling your too-short hair really hard in order to secure it. Click here for tips on safely securing the hair during exercise .
Avoid using clipped in ponytails - the weight of them can - quite literally - tear out your hair.
Always use fabric covered hair bands - avoid bare rubber bands like the plague! And remove bands, clips and ponytail holders from the hair at night - this is when you may be putting your hair under tension without even realizing it.