strengthen your legs?
also, do you have a super noodly board? if you're not getting enough height in your ollie, then you need to use your board to get up.
practice by strapping in and ollieing on a flat surface. before you jump, lean back and load some of your weight on the tail of your board and then jump more with your back foot. If you do it correctly you will use the tail of your board as a spring and you will notice that you will really pop up a lot higher.
again, try it a bit on a flat surface in a room with a lot of room. the first few times you will feel off balanced but it gets better the more you do it.
EDIT:: ahhhhh beat by SNOWOlF as usual