Brace the upper arms on a preacher bench or glute ham bench holding a dumbbell in each hand with the palms up. Keeping the arms supported on the bench, curl the dumbbells up, and return to the bottom under control. Avoid hyperextension at the bottom.
 
Purpose
The preacher curl enforces strict curling to maximize bicep focus, and changes the resistance curve relative to a standing curl. Using dumbbells helps develop or maintain symmetry between the arms.
 
Programming
Generally sets of 8-15 reps are appropriate.
 
Variations
Dumbbell preacher curls can be done single-arm, with a pronated or neutral grip, or supinating during the curl. They can also be done using an incline bench for support.