Brace the upper arms on a preacher bench or glute ham bench holding an EZ curl or straight bar. Keeping the arms supported on the bench, curl the bar 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.
 
Programming
Generally sets of 6-15 reps are appropriate.