As you jump into the air, your front foot needs to roll slightly inward, and with the outside of your foot, you want to guide the skateboard as it flies into the air. Some people describe this as dragging the side of your front foot up the skateboard – that’s more or less what is happening, but what you are doing is using your shoe and the grip tape on the board to pull the skateboard higher into the air with you, and guiding the skateboard to where you want it.
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.