Personally, I like to look at K/9, BB/9 (or K%, BB%) and then HRA and maybe consider inherited runners scored. My thought process is that if I need to bring in a guy in a jam to get out of it, I'm going go for the guy that won't put anyone on base for free, best chance to get outs without the ball getting put in play (an maybe advance runners), and if he does allow contact it should be weak. I find ERA over 30 days for RP to not be overly helpful. A guy could have a sparkling ERA (lets say, only two ER on the month) and good K/BB ratio. But he came in with bases loaded twice and gave up two grand slams. His ERA only gets dinged for two ER, but he's allowed much more damage than that.