I will usually break in my belts on the trail ride up to an area, so nice and easy varied throttle for 30km or so. Will do this with my spare belt to so when I blow a belt my spare is already to go no need to take it easy.
Reason I ask is because I blew a belt with around 800 miles, put 20-30 miles on a belt and was playing in deep snow for about 15 miles and blew another belt. I'm mostly thinking that heat was the problem.
When the belts blow they seem to just be in a million pieces, make sure both clutches are clean. My buddies primary was sticking a bit after blowing a belt so gave it a good clean at home before going out again. Vents help a lot with these sleds.
Good idea to wash with hot soapy water when new. There is a film from the manufacture on the sides. If you have a glazed belt the film was not washed off. Scotchbright work well to scrub sides.
Pop the clutch off and put it in the dishwasher. Cleans all the little fibers and grit out of it. I usually spray a little graphite or silicon dry lube onto the shaft and into the rollers.