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Use your arms upon takeoff to initiate the spin.
Approaching the jump keep a good athletic shoulder width stance knees bent and bring your arms out from your sides in the shape of a "T" almost parallel with the slope. As you get to the end of the takeoff, sort of pre load your shoulders and arms in the opposite direction you are going to spin and then as you pop, swing them in the direction you want to spin. Don't reach for a grab until after the pop and the spin has been set. Using this T-set method you set the spin and then you should have a good balanced rotation and can then reach for just about any grab.
This is what helped me really level out and clean up my spins. The harder the initial throw, the more spin you can get or the more lengthy drawn out grabs. Watch how some of the pros set up for their big spins and mimic that motion on the takeoff. The more level your arms rotate on the pop the more level your spin. A 360 should be extremely easy after this and a 5 is just a matter of holding the grab a bit longer through your spot and then just setting the skis down tails first.