1. If there are two sides, add a third or take one away.
2. Every now and then, change sides: when someone is ahead by two somethings or when someone throws a 9, or when somebody has to go to the bathroom.
3. If there are turns (checkers, gin rummy, serving the ball in ping pong or volleyball), take them together, at the same time, as in "1, 2, 3...go," or every now and then skip a turn.
4. If there is score, keep playing until you discover who's the second winner, and the third, and the next, and the last. Or give each other points, or play pointlessly.
5. If it's not fun, change it: add another ball, or a rule, or a goal, or take a rule away, or change a rule, or borrow a rule from another game, or add a whole game and play them both at once, or do something silly.
6. If it's still not fun, change yourself: try it with your eyes closed, or with your "wrong" hand, or tie yourself to someone else.
7. If it makes the game better, cheat.
(Edited by Major Fun with contributions by Matt Weinstein, Elyon De Koven, and Jon Jenkins.)
courtesy of The Oaqui*