Pull your knees up into your chest – How far you actually want to pull your legs up is totally up to you, but the higher your feet go, the higher your board will go too. By the way, the highest recorded preferred stance ollie-pop is 45 inches, done by Aldrin Garcia, and the highest switch stance ollie-pop is 40.125 inches, by Gavin Caperton. And if you’re going for endurance rather than height, the record for most consecutive hops is held by Rob Dyrdek, who did 215 of them in a row on his TV show Rob and Big. So those give you something to shoot for.
To get higher ollies, I would suggesting resting your back foot on the very end of the tail and the front foot in the middle of the deck to allow for more slide. Popping as hard as you can and catching your board in the groove of the nose with you ankle practically resting in the groove will level the board out in the air. after this initial movement slide you feet back to the bolts on the tail and nose to ensure a clean landing.