Every time we toasted, I simply kept using the same shot glass, never drinking or refilling the contents!
I did something like this at a wedding (I was driving) but I had a bottle of mineral water between my legs. The other guy kept insisting we drink, screaming at me when I initially said I wasn't drinking... hence the water). I kept topping up my glass when he wasn't looking. Of course, as he got drunker, I didn't. Eventually he looked at me with total admiration, "You drink like a Pole!"
Other times I've accepted a glass and kept my fingers around it to hide what was in it, whilst tipping it into a teacup or the nearest flower pot.