In a group, move downwind if possible.

Tuck your head under your underarm (like a duck), press nostril firmly and BLOW. If you are not forceful in your blow, you *will* end up with snot all over.

Those of you who swim for x-training should be good at this; it's a lot like forcing water out of your nostrils under water.

If you are racing or on a fast training ride don't worry about grossing other riders out. At the end of the day everyone is covered with sweat, blood, mud/dirt and GU anyways...what's a little snot???