Moustaches & Masks...

[polysemous1]

Trim it a little shorter. I actually did shave mine off after having one since I was 17 (well, sort of at 17) and it wasn't as bad as I feared and it came back fully after about 10 days. I stopped the shaving though and don't get much leakage. A little readjustment of the mask once I'm in the water seems to do the trick.

 

[Tom Winters]

If you're serious about diving AND keeping your moustache, then you need the Official Diver's Moustache. I never took diving seriously enough to do this, but I fully accepted the ostracism from the diving community that I have endured most of my life.
There is no need for you to undergo this when this alternative is readily available:

 

[Dannybot]

I let a small amount of air seep out of my nose as i dive, works well.