The director of a masterclass I took at Morehead once made us do a
simple deep breathing excercise, that even feels good to do anytime
just to clear your head...Here's how it goes:

1. Exhale fully to clear your lungs of air that should normally
be in there.

2. Inhale about half your lung capacity.

3. Inhale your full lung capacity.

4. Inhale a little bit more until you feel like you'll bust (trust
me, it can be done)

5. Slowly exhale, keeping a steady airstream, using your
normal flute embouchure. **If you have a friend poke you in
the stomach with a pencil (eraser side! no injuries please!)
you should be able to push the pencil back towards them**

So, this exercise is to improve breath support so that when you
play a piece of music as a solo, or even in band or orchestra,
the sound doesn't lose intensity or pitch. I have found this
exercise extremely good to do.

Always sit up and play like you mean it! That will
improve your sound and make it easier to breathe.
Imagine that your windpipe is just a large hollow
tube that you have to push the air through. Keep
it open...imagine that you are yawning.