One of the main reasons behind short belt life is improper adjustment or poor care of the belt itself.
If you use harsh cleaning solutions in the belt area like wheel cleaners, it can have an adverse effect on belt life.

Here's a few pics of a new pulley on the left and a worn belt pulley on the right.
The worn hard coating will chew up a belt in no time when the surface is broken and the pulley should be replaced if this happens.

http://i149.photobucket.com/albums/s45/bills985/Pulleyworn.jpg



Here's a few pics of the worn belt on the left verses a new belt on the right.

http://i149.photobucket.com/albums/s45/bills985/Beltworn.jpg