I definitely know where you're coming from, Julie! While I drink socially, I'm so small and have such a fast metabolism that I have to take care not to get tipsy very quickly. Also I used to be friends with a girl who would try to pressure me into drinking more than I was comfortable with so I had to develop some tactics to deal with this!
1) No means no! This should work on most people but occasionally it doesn't and sometimes you don't want to seem confrontational, but this should be your first option. In this case, if someone buys you a drink, don't even touch it. Give it to someone else to make your point.
2) Set yourself a limit of two or three drinks with two glasses of water in between. This way you will feel really full from all the water and remain sober as well.
3) Only drink when you're eating, and then drink water or soda after that.
4) Water down your drinks, white wine spritzer or spirits with extra mixer.
5) Stay away from drinks that you know will get you drunk. For me, it's gin or really sweet cocktails.
6) Drink pints of beer or cider ever since they take much longer to drink than cocktails and contain less alcohol.
For when the person/ people you're drinking with are being jerks:
1) Be assertive!
2) Be willing to lie- I took antibiotics/ painkillers earlier, drink soda and say that there is vodka in your drink, I have an early start, etc.
3) In my case, reconsider if someone who doesn't respect your decisions deserves your friendship.