Load up like a spring. I find its a combination of timing between flying your kite quickly back over your head (briefly), loading up your edge, and most importantly assisting the take-off by loading up your body like a spring and really pushing off through the legs. Gets me 30 to 40 foot plus with C kites in the right conditions and even higher when using a small wave to boost off (with the help of a couple of old Rhinos). But like the other guys said, practise and more practise. If you want high you will get it with determination. Hope I've been of some help.

p.s Just to create a stir

... never seen people jump as high with bow kites as I have with C kites...