It's a lot easier to get caught out than I ever expected - I was astounded the first 3 times I went out how quickly people started guessing that I was preggers. Here's a few tricks that have proven quite successful since:
The Desperate Housewives Manuevre
(inspired by Lynette & Tom Scavo's business dinner outing, but much more expertly executed)
I told a close and very discreet colleague the truth, and she has been my partner in crime ever since. When we go out for work drinks, we always order the same drink. She drinks quite quickly, and I sip super duper slowly. I put my drink on the bar, she puts her drink on the bar, and when she reaches for another sip, oops! She takes the wrong glass, leaving me with a 1/4 to 1/2 full glass.
It's my round
This can get expensive but sometimes it's worth buying the round, then you can order yourself a mocktail or tonic water with lime and no one will be the wiser.
Toilet Break
I sometimes take my full glass of wine to the toilet with me and miraculously emerge with a 1/2 empty glass. Sometimes I keep a bottle of water in my handbag (sparkling is better if possible) and make myself a nice light spritzer while I'm there.
I think the trick is, never say no to at least 1 glass. If you can convince people you've had 1-2, it's a lot easier to "slow down" than if don't drink at all. The antibiotics trick will work once, but you'll need to prove yourself a proper alcoholic the next time you're out so use with caution. The "already hungover" excuse does work too. Finally, it's always good if you can behave as though slighly drunk already (requires decent acting skills).
Good luck!!!