A regular surgical mask that you see most people wearing will probably be better than nothing as it sort of prevents you from touching your own face, but only if you use it correctly. If you are playing around with it and moving it around with your hands ... I think you get the point.
As the flu is spread mainly through the air, an N95 or N99 mask is more effective but you might look weird and paranoid wearing one of those. I say just make a habit of not touching your face and wash your hands well.
The swine flu might make a monstrous comeback this fall as the Spanish flu did, so maybe stock up on N95 masks just in case?