I didn't see anyone mention what I did, so I thought I'd share it. This solution is kind of luck based, but if you're having a rough time winning a certain game mode, maybe this will be helpful:
Play a game and when the round is nearing completion, look and see if the winning team has an open spot. If it does, quit out of the game, go to recent players in the xbox guide, and join the game of someone on that team. You'll then be on the winning team just in time for the game to end. I did this for both Squad DM and Conquest.