photo via pecdec on flickr
Sit on bench. Grasp dumbbell between feet. Place back of upper arm to inner thigh. Lean into leg to raise elbow slightly.
Raise dumbbell to front of shoulder. Lower dumbbell until arm is fully extended. Repeat. Continue with opposite arm.
The long head (lateral head) of the biceps brachii is activated significantly more than the short head (medial head) of the biceps brachii since the short head enters into active insufficiency as it contiues to contract. (via exrx.net)