Water ramps are sick, not sure if there are any in Cali, Park City is probably best, but expensive if you don't have a hook up. There are also ramps in CO, but still a long way from Cali. Water ramps are pretty good training, but tricks don't transfer directly, but definitely helps confidence in the air, also will improve your technique a lot, especially if you have a coach helping you. Besides that I agree with mtn biking, but the best, and I can't believe no one mentioned this, is diving boards. You have to have a pretty dope setup to do a dub cork on a tramp, but it's nothing off a high dive. In any case, all of the legit skiers I ride with are in stupid good shape, that's probably your best bet.