Like said, you can tumble onto mats. This should help you get higher. You can also do basics onto really high mats. For example, you can stack a resi onto an 8" and put a cheese mat in front of it. (Put it so that the cheese mat has the wide part as the base and the "pointy" end in the air so that it is slightly above the height of the mats you stacked.) Do a roundoff bhs and rebound and try to go to your back onto the mats. First you can start off rebounding to your feet onto the mat.
If you have a trampoline (or a tumbl trak...i'd use a trampoline for this though) start off doing really high back tucks and then progress into your passes. The entire point is to try to wait until you are really high in the air to do your pass and not rush it. You can do this, since you are on a tramp. You will learn hopefully in time how to wait and not rush, and this will get your passes higher.
Why you might not be getting height:
1. You rush! Don't go for your pass as soon as your feet hit the floor. You need make sure that you set and get in the air before you do anything. You don't want to punch leaning backwards, and if you rush, you will lean backwards and this will make the skill "whippy".
2. You let your stomach come out (and your back arch...meaning you're loose in the middle) on your punch! Squeeze your stomach muscles and try to stay as tight as possible. If you need to, work a little bit of extra core conditioning. If you do plyometric drills (punching over panel mats in a row, doing bounces in place, tuck jumps in a row, punches over barrels and blocks), focus on your stomach and making sure that it is tight. You should be doing these kind of drills.
3. You're throwing your head back. As a result, this makes you rush and many other problems come out of this. Try to spot something large (try to make it colorful, as it may be easier) as you are punching that would be directly in the line in front of you.
4. You bend your knees slightly! Like #2, do some punching (plyometric) drills. Make sure not to bend your knees and focus in on this as you tumble.
5. You are s l o w ! ! ! If you aren't getting enough power and speed in your run, how can you expect to have power in your tumbling? What happens if you don't have power in your tumbling, well, you don't get height. Try to speed up your run, but, most importantly, SPEED UP YOUR HURDLE! Make this as fast as possible and try to get the bhs to be even faster. Everything should speed up right before you punch.
Sometimes, it is not always drills that can make your tumbling higher, it is your technique.