Start: Sit up straight at the end of a bench. With an underhand grip, hold two dumbbells straight down, at arm's length.
Finish: Curl the weights toward your shoulders. Stop and squeeze when the dumbbells are 6 o 8 inches in front of your shoulders. Hold the contraction for a beat, then slowly return to the starting position. Don't lean back to help yourself lift more weight, and keep your upper arms against your sides throughout the exercises.
To shift more of the focus to the outer head of your biceps, do the curls while sitting back on an inclined bench. Start with the bench at about 75 degrees, and work your way down slowly. The more dramatic in the incline, the tougher it will be on your shoulders.