I've rarely been asked why I don't drink, my close friends know why and my other social situations all involve mingling around, where noone is keeping any sort of tabs on who drinks what.
If asked, which has only happened once, I just say that I'm trying to lose weight or doing weight watchers, and that part of the diet is not drinking. Its pretty simple to come up with something - I had a big bit of cake this morning so no wine for me now! That sort of thing.