Consistency and reliability go a long way with me. Everyone called me crazy last year when I said Shaun Alexander should be drafted ahead of Portis and even Priest, and that only LT should have been drafted higher. (and it turns out even LT shouldn't have been)
But I try to stay away from RB's that are over 30, have had a history of injuries, or have up and down years (C-Mart, Tiki) unfortunately guys that fit that bill are few and far between, and after LT and SA you're pretty much going to have risk whether it be age, injury, new team, unproven, etc.
So consistency and reliability are the main thing I look at, not just with RB's but with all positions. After that, O-line, schedule, etc. play a pretty even role.
"90% of the game is half mental" - John Madden