Full beard here (well, sometimes it's a goatee, but for diving purposes no diff). Reiterating and recapping from other posts here and my own experience:
1) Find a mask that otherwise fits extremely well. Put it on your face and inhale through your nose: even with the leak around the 'stache, you should be able to keep it on your face no-hands. It's going to leak a bit around the facial hair, all the more important to make darn sure it doesn't leak anywhere else.
2) Get used to constantly exhaling a bit of air through the nose to compensate for the leakage. Note that, because of this, you will need REALLY GOOD defog on your lenses! You will need it far more than most other divers. I tried several - baby shampoo, spit, etc. - and what works best for me is the Sea Gold gel. YMMV.
3) A product that seems gimmicky but isn't:
Dive Right In Scuba's silicone mask sealant. You can even put some directly on your 'stache to help seal - although it will never completely stop leaking. Not too difficult to clean off. Do NOT use Vaseline or any other petroleum-based compound, as the oil in it will eat into the mask skirt over time.
4) Get used to water in your nose, because you can't fully stop it, you can only compensate using the above techniques.