Does anyone know how to get a fuller sound when playing low notes like C and B?

Keep your air stream steady, and think of aiming deep into the embouchure hole, much deeper than where you normally play at.

Of course, being able to do this takes practice. There`s a little exercise you should try every day to help you gain a fuller sound down low:
1. Start at middle D (an octave above the D above middle C, if that makes sense).
2. Hold it out for several counts, while working on focusing the air stream and aiming as deep inside as it takes to gain the richest tone you can.
3. Repeat on C#.
4. Then C.
5. And so forth, all the way down the flute. Be sure to hold each note for awhile. If you have a metronome, put it at 50ish and play whole notes. Make sure you do it every day, or you`ll never get anywhere. Yes, it`s boring. But it pays off in the end.

err... by "middle C" I meant middle C on the piano, sorry!