Yeah trampoline is probably the most ideal way for most people (and especially someone from the midwest) to get their rotations down. I for one, am definitely not going to be able to head out to Windells or get to water-ramps.
I probably spent an average of 3 hours a week on my tramp in the Summer and it really helped me. When I tried taking it to snow, I already had the rotation dialed and it was only a matter of popping more, throwing it not as hard, etc.
And I'm not much of a rail skier, but if I were, I would definitely invest some money and a day or two to build a set-up.