When spinning more than a 3 it gets tough to grab because it becomes foreign to your body movement. Trampoline will only do you so much good, its actually better to tramp without a board to your feet because its such an unrealistic pop compared to actually being on a snowboard.
Just spin your 5's and rodeos without grabs for a while, it will help your body (primarily your knees) understand where they need to be during what position of the spin. Because it is foreign, your knees will tend to be further away from your body, once you feel like you can tweak, shifty or poke during any moment of your spins...thats a sign that your mind and body are completely in control of your motions. This sense will allow you to focus solely on the grab. Backside, I prefer indy and tail. Frontside, I prefer melon and stale.
With backflips, if I am to grab...I grab melon. I go wildcat style when grabbing, or else I just go straight to a layout. Wildcat should feel more along the lines of a cartwheel because of your leg alignment. They flip much faster because you are in a ball, so practice on smaller jumps on a slushy day. Really the only advice I can give.
With rodeos, you shouldn't have to flip really hard to get it going. It will be like the wildcat but even less effort to engage. Once you pop back, grab that melon or that indy and try to land switch. Their definetly a tough trick to stomp. Under rotating means being toe heavy, and that will make you carve or wash out your landing. Heel heavy will roll you to your butt.