I hated how in Halo 3, only the winning team got any experience. I'm not saying the losing team should get the same as the winning team however. If, for instance, in Halo 3 you win a game, so you get 3 points towards your next rank. If you lose, you only get 1. Getting nothing makes the game you just played feel like an absolute waste of time. I mean give the losers SOME leverage. Jeez... Whew, that brought back memories of rage days in Halo 3.
Just make the reward for winning valuable enough players will go for it, and the penalty for losing decent. Neither the reward or penalty should be overwhelming.