Try the mask sealant from Dive Right In Scuba. Besides putting it around the edge of my whole mask, I also apply some directly to my moustache, right under the nose where the mask rests. It has no flavor or odor and is silicone-based, not oil-based, so if anything it will actually help keep your mask skirt supple.
I have also noticed that, ironically enough, it is easier to keep a good seal over my moustache when I let it grow out a bit, as opposed to keeping it short and stubbly. I think that's because it's easier to maintain a seal when the hairs are laying down, as opposed to sticking out. But the Dive Right In mask sealant definitely helps.
+1 works for me too.