It looks simple but yet complicated and vice versa. It takes most weeks to months to learn n some even years but a lifetime of effort to be consistent and be perfect at it and able to go higher, faster and further. It is not a feat you achieve once and it can be thrown out of the window if you want to continue to skate well. I've know skateboarders who just does ollies, boned ollies, ollie north and ollie souths, fakie ollies, switch ollies and nollies and nothing else but everytime he's popping them over 3ft height and up ledges, over gaps and down stair sets or over handrails. So no one's gonna argue on that even though he's not bustin some new fangled pressure flip late flip revert or something. He gets mad street creds for those huge ollies.
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.