When practicing long tones on the high notes (I assume you have already been spending a lot of time on this if you are working on dynamics...nod/wink) you should concentrate on two things:
- First is to have a very focused airstream...relax your lips and think about where you are directing the air...sometimes it is useful to just blow into your palm (without the flute) and feel your airstream. A good exercise for this is to hum the note while you play as this will limit your air volume and force you to focus to get any sound. I still practice scales while singing along in my throat as a way to force efficient air usage.
- Second is to vary the dynamics as you hold the long tone - fff-->ppp-->fff. Take the note down to a subtone if possible
let us know about progress...the same exercises are useful for playing low notes loud.