The key is to develop the ability to pick out scum at a rate high enough to be useful (i.e. significantly better than completely random). You will never get all the scum all the time, but if you are right often enough, people will begin to pay attention. As scum, if you have developed your scum catching abilities sufficiently, obviously that will help if there is more than one evil group, but even if yours is the only family, your method (whatever it may be) will begin to provide you with reasonable innocents (i.e innocents that you would probably pick out as scum if you weren't scum yourself).
As scum (and keep in mind, I've only actually played as scum maybe once or twice in the past year
), I tend to dissociate myself from the family as much as possible. I let them make decisions on who to kill, etc. Then I treat day as though I was just my usually sparkling innocent self. I pick out scum, generally without regard for the fact that I actually know who the scum are. Or, more often, someone attacks me and I defend myself and counter attack (it's easy to get away with an innocent lynch if they attacked you first). I am confident in my ability to be a very difficult lynch. That isn't to say that I don't give *any* input to the family. If there is a key decision that I feel needs to be made, I'll let them know (i.e. I think someone is a Doc or whatever). And whether I push for the lynch of a fellow scum obviously depends on the game situation (how many we have left and such). Also, this by nature can not work for everyone. Someone has to make the decisions, after all.