A rather non-traditional drill I use for carthweels on beam is to take a folded pannel mat (like a 1" folding mat, folded up) and do a cartwheel where the lunging foot is on the mat. Yes, you want to do the cartwheel going long ways on the mat, not short.
This way, it forces you to actually push with the lead leg to get up, keeping you in more of a handstand line, isntead of a roundabout cartwheel. Then, transfer that lunge and drive to the beam.
The more you work on keeping straight through the cartwheel, the easier it will be to spot where your feet need to go. Once that first foot comes down, spot the end of the beam and push with your hands - stand up, and voila! a cartwheel.
Hopefully that will help you.
Good luck!
Ryan