You're not really looking for dance training, you're looking for flexibility. First of all make sure you push yourself on the flex training you do at the gym. When you warm up, make sure your splits are down and square with pointed toes and straight legs! Seems obvious, but just making sure you train that simple position well will make it easier to hit in the air. When you do oversplits, get your shoulders back (this will help with back flex also). If you don't do oversplits at the gym, do them at home.
I can recommend some active flexibility drills as well as other things.
For back flexibility and hip flexibility
- oversplits with your back leg on the mat, not your front. Put your knee on a mat, facing away from it, and go into a split. Try to touch the mat behind you with both your hands.
- Double bent-knee against a wall. Do a deep lunge with your back foot bent up against a wall, like a double-stag position. Then arch back and try to touch your head to the wall or to your leg.
- Ring split against a wall. After doing the above stretch, slide down into a split with your leg bent against the wall. Make sure your leg is pointing straight up behind you - it's okay if you can't go quite all the way down. Arch back here too.
- Find a clear doorway in your home. Now go into a split with your front foot at the base of the doorway and your back foot sliding up it. Grab onto the other side of the doorway and push. Try sliding your front foot out so you are in oversplits and pushing against the doorway to arch your back.
Active flex (you can also do all of these with weights on your ankles)
- Kicks. 2x25 kicks forwards, backwards, sideways.
- Kicks lying down, same as above. You can do the backwards ones on your knees
- Ring kicks and needle kicks, 2x15. You can also do ring kicks on your knees.
- Lying on your back with your legs in the air, moving quickly to a straddle and back up again. 2x15
- Split jumps on a trampoline, 2x15 each way including straddle. You can also do switch-split jumps here.
I'm sorry if I over-explained some of these, I'm not sure how much you already do.