Yeah, I know-- I meant that it will take a lot of extra effort to make the high notes sound full.
One situation where the pushing against your diaphragm won't work extremely well is when you need/want to vibrato. When you do-- you'll use your diaphragm, and for extremely high notes (just about the 4th octave), your vibrato will sound weird. Kind of like "thin sound, full sound, thin sound, full sound" when you push and relax etc. Other than that, yes, exerting pressure on the diaphragm is the best way to get over thick lips for high (actually, all) notes on the flute.
Aria, you sound a little taken aback by the comments about the complicity and stamina needed. The only reason why the flute needs more stamina than the other instruments is, half (or a varying percentage) of the air you project out doesn't go into the instrument because you're blowing across. With that being said, there is not much (if any) resistance when you blow across the tone-hole, meaning that you can very easily get out of tune.
This is where TKbonz's suggestion of applying pressure to your diaphragm makes sense. How it works is basically constricting your muscles after you've breathed in, and not allowing the diaphragm to slack. This will (hopefully) make your projection of air more controlled because of the inner resistance. That's all there is to the stamina part really.
Oh, and the easiest way to start learning the flute is to go buy one. Go read up books on flute playing, and then do exercises with only the headjoint. After that, try playing simple melodies like nursery rhymes. I started that way in my school band as well.
Cheers,
Zexun
Oh yeah,
Flutes are awesome for photographs
(Shot with window light and bounced popup flash)
Powell Aurumite Rose-Gold headjoint. Each headjoint costs about 8k. I'll surely miss the feeling of playing on this when I graduate from my school =(