The one we discovered in MW3 was to change max players needed in the lobby to start the game.
The other was to find a game setting (dvar) to change the game mode to private match, and when the intermission timer goes you change it to a public one. (Allowing you to change settings and such for the game as it were a private match, but you gain xp and such)
Also there's a "force host dvar" if I don't recall wrong.
-And some are able to use the game console to start the game as host (as done in MW2 with a tool many used to mod game matches)
This is found by using tools like IDA and others. I've just used IDA