Lots of drills and even spot. I think spotting a gymnast when learning how to do a cast handstand helps them realize the timing and technique of the skill. There are also many strength drills that can be done--for instance, kick to handstand on a floor bar then lower down to planch (hollow body and feet off the floor) then raise back up to handstand (with a helper of course).
If she is doing it on her own, have her try thinking about driving her heals so hard that she goes over the bar, or to planch more. One drill I like to do is: set up a big resi mat on one side of the low bar, then have the girls planch (big time planch) and just fling themselves over the bar onto the resi mat--I find this to be a fun drill that helps the girls get used to planching.