Personally I'd just avoid them well, and be ready well in advance for any party invites. Never joining any occasion where alcohol is nearby or served in any amounts. That's the easiest case. If you can't avoid confrontation then you can simply say 'No', but without getting upset yourself. And if the person/people try to talk you out of your decision just leave. The idea is not to get upset or confused, because I find that during such a moment it is easy to make a hasty and big mistake.
People who are bothered by your decision not to indulge in alcohol are not very good friends, and highly likely have a problem with alcohol themselves. I'd just avoid them and try my best not to get upset at them for any reason.