There are a lot of things you can say. Sit down and write out (honest) answers that don't go into it.
First of all, no one has to be given a reason. It's simply no one's business.
"I'll have a [insert your beverage choice.]"
"Why aren't you drinking? C'mon, just one. Everyone is drinking."
"No thanks. Soda with lime, please."
"Come on, just one!"
"No, thanks. I'm good."
There are a hundred ways to say "No." Just keep repeating it. Actually I've only had one person ever go further than my first, "No." Just be committed. No one cares as much as we think they do.
