heres what i know , and it works and its what the pros do hahaha . when u ride up to the jump, depending on whether you want to spin frontside or backside , you either carve on your heel edge or your toe edge when you take off, so you start off spinning . Also you might want to slightly wind up your body and release in the direction you want to spin as soon as you leave the jump . i recommed starting small, with 180s first, try them frontside because it tends to be easier for most people but thats tottally your preference. also before you even start jumps or spins practice ollies .
this reply is maybe a bit late for your 09 season but I tended to practise by hitting small jumps and go for maximum rotation 520ish - the worst that can happen is a wee slam - no pain no gain.....michal is correct in winding up the body first but stay loose. Start your rotation of your shoulders early on the jump and you will naturally be in the first phase before going airborne. the landing is easier on a downslope even if only slight.