Tips on how to ollie on a skateboard?

Ive been practicing a lot. I know the basics, one foot in middle and other foot on tail of the board, bend your knees and pop the back of the board as hard as u can. i cant get the back wheels of the board up very high, i slide my foot to the front and bend knees on flat solid ground. how can i ollie, any tips or pointers? Please help i love skateboarding and if i could ollie ill soon be able to do other tricks. thanks for any help