Look underneath your armpit in the cartwheel. Follow your hands to the beam in the BWO. Again continue following your hands to finish instead of just lifting the arms to finish.
Keep the body in alignment as much as possible. Don't break the shoulder angle, don't put the head in too much or out too much. Keep a straight line from fingers to heels in the lunge and keep your stomach/back stiff and not loosey-goosey.
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.
Cartwheel on Beam...Stand with legs together and arms above head. Step and place 1st hand across the beam with fingers pointing out bring 2nd hand straight over top of the head and place on beam with fingers pointing inward toward 1st hand (letter "T" position) and follow through with legs straight over and not around. Start this drill on floor or floor beam. This technique will have your hips rotate as you bring your 2nd hand over so you will land straight on the beam facing the way you started. Land with your feet one directly behind the other and when you can do that consistenly then you can adjust the back foot. If you land with the back foot already adjusted your hips will not be square and you risk falling. This is also in my opinion the proper hand position for roundoff and your tumbling passess will be straight. If a tumblling pass is not in a straight line it is always the fault of an improper roundoff.
Back Walkover on Beam...It is my belief that on this skill the hands should be placed one behind the other. If the hands are placed side by side there is a greater risk of injury because one hand might slip. Putting the hands one behind the other will also keep your hips square.
BACKWALKOVER: watch your hands ALL OF THE TIME. Go slowly and don't rush. Concentrate on tightening your arms, shoulders and lower back and pointing your toes.
When your hands touch the beam that's when you start using your second leg, if you get the timing wrong its hard to get your legs over. Once your first foot has landed use your hands to get the final push.
CARTWHEEL: I don't know what you're doing with your hands but I was helping to coach a group last night and the coach was teaching them to put your first hand sideways across the beam and your second hand is longways with your fingers pointing towards your first hand. It kinda looks like a T. Then cartwheel, making sure you point your toes. You should just be able to see your first foot landing. Using your hands like that makes sure you'll rotate your hips, I had a go myself and it worked great. Once you move onto cartwheel roundoff this will be VERY USEFUL.