Sit with a dumbbell in one hand and allow the arm to hang straight down inside one leg. Keeping the back of the arm against the inside of the thigh to eliminate any cheating, perform a curl, then lower under control.
 
Purpose
The concentration curl is a way to totally isolate the biceps and perform a curl without any assistance from the rest of the body.
 
Programming
Generally sets of 6-15 total reps are appropriate.