My children and I have made this something of a game. (For privacy's sake, I will refer to my children by only their first initial.) If my son is sad because he is not doing well in a game we're playing, I will tell him how much more fun the game can be if everyone tries to have a good time, and I'll say, "I need a happy T. Where's my happy T?" Usually he can manage to push out a little smile. If not, then I have to start looking for "my happy T." I'll say, "I know there's got to be a happy T in here somewhere. Let's see..." I then proceed to check under his arms, poking around in his armpits, under his chin, around his middle, until he's laughing so hard from the tickling he can barely breathe. Then I say, "Oh look! There he is! I knew I could find my happy T in there!"
This is one of the best ways I've found to turn a potential tantrum into smiles and giggles. And that makes the day more fun for everyone!