I think many people simply work Bi's way to much, they are a small muscle and get hammered if you work back intensly. I myself do 1 sometimes 2 exercises for my bi's. Here is what I have been doing lately. Dumbell concentration curls, elbow on inside of thing, slightly bent over. I use 90% of my 1 rep max weight and do 2 sets with a 3-5 minute rest between sets. Then I drop to 80% of 1 rep max and continue to do sets. Each set I do 5 reps and stop (even if I can do more) with a 30 second rest between sets. Once I cant get 5 reps im done with the exercise.
I am cursed(or blessed depending on how you look at it) and I have long biceps that go elbow to shoulder with very short tendons. This means that I dont have any real peak when flexed, but have a very strong bicep.