Yes, many good posts have been written on this. Research your agent. Choose him specifically. Let him know you looked into his track record. Good good good.
Here's the thing: I'm not exactly sure how.
I think part of this is that I write high fantasy. Honestly, choosing an agent is less of a "I loved this book you repped" and more of a "you haven't specifically asked for no queries from my genre!" Yes, I have a couple dream agents, a couple big hopes, but I'm too nervous to query them in case they say no - silly, yes? Silly.
So often I query an agent when I haven't read their books. I can research the books a little, but I feel like that's cheating. An impressive record is good to note, but not especially awe-inspiring, as far as personalizations go. What do I have left?
I'd feel a little creepy saying, "I read in an interview you like cordon bleu! Me too!"
I'd feel a little weird saying, "I want to query you because I liked your website!"
I'd feel a little bit like a jerk saying, "I want to query you because you won't say no based on my genre!"
So what do I do? Any tips?