I do biceps twice a week. Usually two exercises per workout, so four biceps exercises per week. And at that I usually only do 4-5 sets per workout.
Heavy barbell curls once per week and everything else is for definition. I get a lot of biceps work through heavy back work, so I don't think I need to go super heavy on curls all the time. The best thing is that this works for me! My biceps are a little more peaked than they used to be and they're way more define. A lot of it has to do with technique:
Workout 1 (Tuesday)
Barbell Curls 2 x 6-8
Incline DB Curls 2 x 8-10
Workout 2 (Thursday)
Incline DB Curls 2 x 8-10
Standing DB Preacher Curls or Concentration Curls or Cable Curls 2 x 8-10
... Or something like that. Anyway, some extra techniques you can employ if you don't already, is to supinate the wrist as far as possible at the top of the curl (pinky finger higher than thumb). Give it a squeeze and lower slowly. With curls, be as strict as possible. I see so many people loading up on weight and doing them ****ty, and it pays off just as crappy for them. Strict, slow on the negatives, squeeze your biceps at the top (don't let them go loose), get a full stretch at the bottom.
Also, after your sets, pronate and supinate (turn your wrist in both directions) a few times, then flex for like 10-15 seconds (one function of the biceps is to aid in wrist supination). After your biceps workout, do a series of contractions (flexing) and relaxation, and some stretches afterward. Flexing a lot helps bring out definition. I wouldn't have believed that a year or so ago, but it works for me.
Oh, and for the record, if you train your back right, your biceps will follow. Pullups, rows, and deadlifts especially.