I start with
"no thanks"
oh come on just one
"no thanks, I don't want to"
why not, just one
"I have no reason to drink"
it tastes good
"so does club soda"
it's fun
"I don't need to"
It usually doesn't get this far. No thanks usually works. If it doesn't I am patient and can continue with as many refusals as it takes because I know that alcohol is patient and will continue once started. Evetually the pushers get bored with my patience. Once they see it doesn't get a rise out of me, they move on to find a new victim.