trampoline definitely helps. We would also carry her through the jump while saying "jump", it take a lot of repetition but then we would place her on something not too high and tell her to jump, then maybe give her a little push (since it wasn't too high it's ok if she fell) or keep on carrying her through the jump and saying the word. I'd say by about 3yrs she was jumping on her own and now that she's almost 4 she wants to jump off everything!

One way that worked for us was a mini kid's trampoline or even a Wii Game (Sports Edition Game). It woo'd our son into it pretty fast. He still struggles to a point now and again, but overall we were pleased!