We know what you're thinking. You're saying to yourself, "Duh! Like I don't know how to do dumbbell curls!"
Well you smug, lop-eared punk, maybe you don't.
Sure, sure, you know how to grab some dumbbells, sit on an incline bench, and curl the weight, but truth be told there are a lot of nuances to the movement that you might not be aware of.
First of all, the purpose of the movement is to recruit the long head of the biceps, which, believe it or not, is often inadvertently neglected by most bodybuilders because of poor form.
In order to efficiently recruit the long head of the biceps, you must keep the elbows drawn back.
Furthermore, the lower you can adjust the angle of the bench, the more the movement recruits the long head of the biceps. In fact, if your rotator cuffs are healthy, adjust the back of the incline bench to 30-degrees or lower.
Begin the movement with your arms fully stretched out. Now curl the dumbbells up simultaneously while keeping the elbows directly in line with the ground for at least the first 90 degrees.
After the first 90 degrees your elbows will begin to come slightly forward, especially if you're using gargantuan weights. That's okay, though, as long as you kept them in line for the first 90 degrees of the movement. Keep the palms supinated at all times so that the elbow flexors are well stretched.
To recruit the long head even more, bend your wrists back all the way (so your fists are pulled away from your head). This will take the brachialis out of the movement so it can't assist the long head of the biceps.
Once you begin to fatigue, pull the writs back into a neutral position and keep curling until you fatigue or reach your target number of reps.
Oh, and if your head comes off the bench no matter how hard you try to keep it down (as it seems to do with the vast majority of lifters), roll up a towel and place it firmly between your neck and the bench.
You'll find that it increases your strength significantly.
In fact, try it on a fellow gym rat. Just stare at him for a moment, furrow your brow, and then stick the rolled up towel between his head and the bench. He'll think you're some sort of weird sorcerer/exercise physiologist when he pumps out a few more reps.