Your instructor/coach may pair you up for some or most of your drilling and rolling. This post is for those other times when you have a "choice." Many BJJers seem to have this idealized picture of the "perfect training partner" - they would literally spend one of their three genie wishes to aquire this mythical beastie.
I have a slightly different take. I'm not looking for a perfect training partner. One perfect training partner isn't going to cut it. Here's why. Until a fairly advanced level almost everyone really only has one "style" (aggressive/grinder/tactican/loose/tight) or "game" (wrestiling/judo/guard puller/closed guard/half guard/knee passer/standup passer). This means the available training partners at your weight may only be presenting you a small subset of the things you need to work against.
It is vitally important that you have a good partner "mix." It is crazy to train with the same guy/girl every practice. When you hit the mat you want to line up your training partners for maximum effect. You don't need to roll with a dozen different people every practice. You might work it so that you "target" rolls with two or three different guys on Monday, "target" two different guys on Tuesday and Wednesday, and "target" a different group the end of the week. (and of course it depends on when you train and they train. If you typically train on M/W/F and only see the same guys each session try and throw in a T/Th or Sat. session when you can to expand your opportunities)
If you are like me, your favorite person to roll is usually the guy who gives you a good fight but that you can still beat fairly handily. It is only natural, but that is likely NOT the guy you are going to face in competition. Work with guys that stop your favorite takedown/throw so that you have to work alternatives or the guys that usually take you down so that you work your defense. If you are a guard puller, work with guys who really give you trouble and pass effectively. Be proactive to make sure you get a good training partner mix every session and across sessions. I'm not saying you need to get your backside handed to you every roll but make sure you are not coasting and ducking the guys that give you a hard time.
I want to get rolls in with guys who are heavier/stonger, lighter/faster, more advanced, less advanced (do you see a trend). Even on nights that I roll everybody, I try to have a strategy/plan of what I want to work on with each opponent. I will try and share some examples in future posts.
Until then, take control of your training by getting a good mix of partners (make sure you include a healthy dose of guys that frustrate you or take you out of your comfort zone) and try to have an idea of what you want to work on with each one.