The best drill that i have found to help with teaching the not so horrible dive roll is to get a gymnast to stand next a box top (like the old school vault boxes..simply cause that is what we have, the object should not be moveable for the gymnast). The height of the box should be around hip height.
The gymnast start by standing next to he box (arms up to ear, or back whatever arm action you require). The gymnast pushes up onto his/her toes and keeping the belly button pulled in and bottom squeezed, the gymnast falls over and kicks/lifts the heels and flips over the box to a handstand. This needs spotting at first to get them to be confident, but after a while they are away laughing. The best thing about it is that very quickly (with correct coaching cues) the gymnast will learn when they pike, drop the head or arms, go loose, are not squeezing their butts etc..
I have had really good success with this drill.
On top of that i have them dive roll over a obstacle like the foam tower described above. And spotting it for shaping. However the drill is by far my best weapon haha, ontop of conditioning.
Its also important to remind the gymnast to learn to keep head up, look at thumbs, flip go up and flip..
I HATE the name DIVE ROLL as the last thing we all want is to have anyone diving. I call it stretch jump flip to handstand haha. i need a shorter name.
Hope that helps, please let me know how you find this drill if you (anyone) uses it