I always look at % players per flop, with a minimum average pot of 10xBB ($1 at $10NL). Other than that, I try not to sit down left of a shortie unless there's a full stack one to his left. You never can tell if these guys are nut campers or just complete donks.

Other than that, if specific players are giving me problems, I'll do one of two things. I either leave the table, or shut down others so I can concentrate on him for a while. Sometimes I may think he's really nitty, but his line is throwing me off, or he may be hyper aggressive and I need to pay close attention so I can get his stack at some point.