If you push off the top of the lip you tend to go really low which is good if you're going very fast but it isnt really jumping.
Totally agree DT, with that and also the pulling up on the bars bit. If i'm going for it i'm generally pulling up like frig. I'm not just pulling though, combined with a bit of a push (to get the distance - more push equals more length) and a bit of tuck for more height.
OP - Its hard to explain jumping in words without doing the actions, but if it helps, i'm doing them now.
mudfish - lower your weight into the bike (just like you'd do if jumping on the spot without a bike
Practice off the bike too. Roll your hips backwards and down when pumping and forwards and up when boosting off. Think circular rather than up and down movements. Works a treat for me.
Thing is, all that ^^ takes good timing and that only comes with practice. Lots of practice.
If your just learning keep it simple. Get used to how it feels being weightless and try to keep loose and centred over the bike in the air. It doesn't take much body movement to affect your trajectory as you've probably discovered .