Well, normally I let them get on with it. It's probably not the best thing, other will come along with their fab techniques and show me up for an idle mother but I quite often let them carry on and go and do something else.
I used to tickle them, or wibble their lips when they were younger [evil]
If they meltdown over the wii, I turn it off and say "It's supposed to be fun, if you're not having fun, there's no point doing it."
Sometimes I copy them
Sometimes I say, "you're not doing it right, lay on the floor...now kick your legs...now scream IT'S NOT FAAAAAAAAAIR" double
Mainly, I find the thing to try to do is stay calm and ride it out.
If they're having trouble with something, I'll say "If you want help, I'll help you, but not while you're acting like that" then if they calm down, I'll help. If they want to carry on banging themselves about, fair enough.
Sometimes I get pissed off and I say "Oh for god's sake, if you're going to carry on like that, just go and do it in your bedroom."
Stuff like that, really.