Slam your back foot down on the tail of your skateboard as hard as you can. At that moment, you want to also jump into the air, off of your back foot. This part is key and takes practice. The trick is in getting your timing right. You want to slap the skateboard’s tail down, and as it hits the ground, jump off of that foot into the air. Make sure to pull that back foot high into the air. It's a quick, snapping motion.
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.