I have had to change my timing belt twice within a year. What could be the cause?

First, check that none of the teeth that the belt is riding on are worn. Worn teeth will dig into the belt and cause premature wear. Then, check each gear's bearing. The belt tensioner bearing is the most likely one to fail first, but it's usually the easiest to check and fix. With the belt off, spin the belt tensioner (which is usually the plain, smooth wheel on a spring loaded arm; the smooth side of the belt rides along it). A bad bearing will not spin as easily and/or will sound rough. Finally, check for rubbing. See if anything is rubbing against the belt, causing it to fail.