Are they learning it with straight or bent arms?
I teach it with straight arms - I start off making sure they can do a good backward roll to pike, with straight arms, then get them to try backward roll to front support with straight arms. When they have that, I start to work on going to handstand. Firstly you have to make sure that they know when to push their feet upwards. A handy tip I heard a long time ago and use every time I coach this skill is 'as soon as your toes are above your nose, push them up' If they wait too long, they end up missing handstand, if they don't wait long enough, they arch their back.
It is also important that they understand how to open their shoulder angle, so any drills/conditioning exercises with this action will be useful.
Practice doing the backward roll to handstand down a slope and do hundreds with support.
Explain also that it is very important to have good body tension throughout the skill.
Hope that helps,
Marie