Fact: If your upper arm development has stagnated, then hammer-grip curls (palms facing in, not up) can reboot your progress by targeting the often under-developed brachialis muscle. If you've already taken care of that, then here's an exercise from TC Luoma to take it to the next level.

Incline Hammer Curls
  1. Adjust an incline bench so that it's at a 60 to 70-degree angle. Get some dumbbells and put your chest on the pad of the bench.
  2. Allow your arms to hang straight down, then curl them up so that the dumbbells touch your shoulders. Remember, use the hammer grip – palms facing in throughout the lift.

The secret is to allow your arms to straighten out completely at the bottom of the movement. Since you can't recruit the front delts with this exercise and it's impossible to use body english, this is more of a "pure" biceps movement. It'll also help recruit the long head of the biceps, the most neglected head of the muscle group.

Related:  6 Biceps Exercises You Should Be Doing

Related:  The 1000 Rep Arm Workout