My Polaris 330 trail boss keeps breaking the drive belts I put on it. Why?

 

I'd check the motor mounts to make sure they're not broken and putting stress on the belts. Plus if you're using after market belts rather then oem could be another reason,along with making sure the clutches are aligned.

 

A simple problem can be driving in high range when you should be in low range. I don't remember if the Trail Boss had a high and low range or not. If it has low range use it when you are riding an area where you are constantly under 10-15 mph, going up hills where you can't just bomb up them, going through mud, or snow. My son blew his belt on an ATP 500 by keeping it in high gear all day even when in the slow technical sections.