One thing I learned way back at ski camp:
In the middle (flats) of the pipe is when your knees are bent the most. Then as you are going up the wall you extend your legs until they are straight, at the lip of the pipe (sorta like 'popping' but more gradual). This helps me for some reason.
Also, just make sure you can hold an edge all the way to the top. Don't be spraying snow.
And as someone said before, just keep at it.