I think the best thing you can do for yourself is to take conditioning seriously during the offseason. It sucks when you are out of shape and trying to get back into snowboarding early in the season. I've gone with friends who are out of shape and couldn't even last the whole day (what a waste of a lift ticket), and had to wait in the the lodge for me.
I basically just run a lot during the offseason. Maybe about 3-4 times a week, four miles a pop.