In 1982, while competing in the Rusty Harris contest in Whittier, California, Rodney Mullen debuted an ollie on flat ground, which he had adapted from Gelfand's vertical version by combining the motions of some of his existing tricks. Mullen used a "see-saw" motion, striking the tail of the board on the ground to lift the nose, and using the front foot to level the board in mid-air. While Mullen was not initially impressed with his flat ground ollie, and did not formally name it, he realized it opened up a second, elevated plane on which to perform tricks.
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.