Hands-down, the best thing you can do, is wait for them to say anything, and just start laughing out loud at them.
It can be hard to get started, but once you begin laughing, even dishonestly, it comes a little easier. Then you just let yourself double over as you laugh harder and harder. Point at them as you keep laughing, pretending to try fragments of a sentence before exploding into laughter again.
Eventually, you'll be laughing so hard, that you'll actually be a little happier, and they will be *extremely* uncomfortable, trying to figure out why they're so funny, or what you know that they don't...works every time.
Maybe try with a friend first just to develop some sort of technique.
This is absolutely evil.