I forget where I read it, but I recall Terry making this point about relaxation. When your muscles are tense, they push oxygen out. When they relax they take in more oxygen. So it's important to focus on relaxing all your muscles, and you will find less sinking, in general.
I think it's somewhat unhelpful to think in terms of floating up though. I like to accept the fact that I sink... to some depth. The trick in superman glide is to sink your front and rear to the same depth, evenly. This is balance we seek.
I recently had a video made (too chicken to post), and one thing that was really interesting was the push off to start. I'm zooming along flat at the surface, my toes pointed, my heels right at the top of the water along with the rest of me. As soon as I pull my left arm out of the water during recovery, and begin rotation, my body appears to sink 6-8" like a submarine going underwater, and my recovering arm is the periscope.
I think that's just the way it goes, hence the term Total Immersion. We don't talk about it much here, but it really is key. If you don't spend energy staying at the surface of the water, you have a lot more to spend moving forward.