Actually, you do not want to use your head at all, and I'd strongly reccomend attempting a backflip off of a diving board into some water to start with, seeing as how doing a back flop or belly flop is slightly less damaging than a broken vertebrae.
In any case, doing a back flip (or back double) is all about your arms. As you're jumping, be sure that you're circling them so that your hands are brushing past your thighs at the bottom of the jump (when the tramp is depressed all the way) and up to just shy of vertical (think 11:00 on a clock face) as you're jumping off. This will get you the most height possible.
Once you have mastered this, the next step would be to circle your arms past that point (awkward, I know, but this is what starts your rotation.) You want your arms to circle past your ears (to approximate 1:30-2:00 on a clock face) to get this rotation started. Bringing your arms past your ears will cause your shoulders to drop back and this helps with the rotation. Once you have started your rotation, bring your knees up to your chest and pull them in. If you bring your chest down to your knees, you will STOP your rotation and not complete the flip. Once you are in the air, and are past vertical, start looking to spot the landing so you know how long to stay in the tuck. Then come out. Done and done. Once you've mastered tuck, try pike and layout for some added difficulty.
Oh, and a pair of brass ones wouldn't hurt either.
There are many extra things that I could add to this but are much easier demonstrated than explained. I've been a diver for 11 years and am now coaching, and doing flips on a trampoline is very similar to diving (and we use one to warm up on/practice on sometimes). Good luck.