Here is the line that I rehearse before I go out to a holiday event:
"No thanks, but I'd love a cup of tea" -- that's for declining
"No thanks, but I'd love a glass of mineral water with lemon" --
that's for declining everything else.
I find that as long as people can give you something (in my case, a
glass of sparkling water or a cup of tea, they're quite satisfied. But I
DO have to practice those lines.