Sit down on an incline bench, grab a pair of dumbbells and hold them with your arms extended and relaxed in the "hammer" position. This is your starting position.
Raise one dumbbell up as far as natural movement permits and then lower it back down until your arm is almost fully extended. You should always keep tension in your biceps. Then switch and do the same with other side. Repeat entire movement for the desired or prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Don't extended your arms completely when you lower the dumbbell(s)
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or use heavier dumbbells.
This is one of the most effective exercises for the lower head of your biceps. On this exercise the biceps gets a good stretch and the angle combined with the weight forces the biceps to develop into an elongated shape which gives the biceps a more complete look.
The key to this exercise is really the pace. Not rushing it will have much more effects than if you attempt to lift the dumbbells by swinging them up.
Also keep in mind that the angle of the bench is important. If you lower past (less than) the 45 degree angle you may be putting too much pressure on your elbow joints and shoulders. Thus, make sure you do not go to far back down when you adjust the bench.