The skater can gain greater clearance from the ground by jumping higher, popping faster, sliding the front foot farther forwards (starting the jump with the front foot farther back), and pulling the legs higher into the chest to raise the feet higher. Skaters attempting record-setting ollies even contort the legs so that board and feet are not directly below them, allowing the board to rise at or just below the level of the pelvis.
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.