I'm very good in ollie when I was skating. Although it looked
basic & simple, ollie will take years to refine & improve on...
I see lotsa skaters who cant ollie well after years of skating.

The difficult part is the lift off of your back foot immediately
after tapping... everyone can tap, but not a lot of people
can tap hard & lift off from the tap in a split second... the
faster u lift up after the tap, the higher u can go... keyword
here is 'spontaneous reaction' after the tap. There are other
parts of the ollie which is important like the pull, the flatten
out, the hanging time, the landing, etc... but I leave that
for you to discover on this video below... have fun

http://www.youtube.com/watch?v=OFXm3ETUG6s