i reccomend that you try doing flippy spinny things into pools and on trampolines before you try it at you local hill.
im also going to add to the post above me. for each additional X your clothing is, you get twice the steeze. lets say kid A is wearing XL geat and kid B has XXXL. kid A lands a smooth cork 9 and Kid B only does a 360. regardless, kid b is 4 times better because of his clothing