Jimbo,
Obviously, your question is not as bizarre as you thought. Based on my personal 'full-bearded' experience you have already been given a number of good suggestions, several of which I presently use.
As per ID's suggestion, shave the 'stashe a tiny bit under the nose -- just enough to let the under-nose mask skirt contact clean-shaven skin and not facial hair. Depending on your mask (and how thin you wish your mustache to be) you follow the lower mask skirt line over to the smile line. Be sure to get the smile line clear.
And, as Scubabunny suggested, you can lightly coat the mask skirt with silicon grease to improve its seal. One caution, though. If your mask has a clear skirt, the silicon grease will gradually cloud or discolor it.
If that's not a problem for you, you'll find a tiny film of silicon grease will improve the seal under the nose and around the 'smile line'.
Good luck.