Multi (2<) Player "Faction/Scheming" Games

[Hupp n at em]

So I've been pondering how to adapt projects that work fine with generally 2-4 players (me and my friends) for groups twice that or more (my dad's gaming club). Specifically, This Is Not A Test is my Post Apocalyptic warband game of choice at the moment. It works great for 2 players and decently enough up to 3 or 4, but I really don't want to put on a game for 8-12 players where people sit around for a while.

My goal is to run this type of game - everyone has 4-5 figures, nobody is formally allies, everyone has separate, diverse, and secret objectives. I've seen these games run at cons and even played in a few and they are always a blast, what with everyone scheming and joining together only to backstab each other in the end.  But I find my memory is failing when I try to recall the actual mechanics of how they worked, or how they handled activations in such a way that enough people were engaged at a time, and  frequently enough, that nobody gets bored and everyone had a good time. 

I thought, especially with the large pulp/adventure contingent here and the people who run games at BLAM, that you all might have some general suggestions on running these types of games. Is this just a question of picking the right ruleset, or are there general "hacks" you can use on any game?