I've been doing some swimming conditioning to get 1st class swimming to avoid the stupid yearly swim quals.
Anyway, one thing I've learned is that men just suck at floating. It just can't be helped. Especially if you're like most military types, made up of very little flat and shaped like a V. You're gonna have to struggle to stay on your back. That' just a fact.
If you meant you keep sinking on ure back no matter what u try, here's some tips.
When you're swimming, floating shouldn't be a problem as long as you keep your chin up, lungs full of air and of course keep those legs kicking. Don't kick harder than you can handle or you'll cramp up.
For just a dead float, like someone mentioned, if you've got air in your lungs you shud stay up. When exhaling, use your arms to keep ure upper body up, stroke those hands downwards at your sides. Then exhale a nice deep breath and keep it in.
This is just for skinny dipping/bathing suited. I honestly don't know how well this would work in a flight suit.