Grip width
 

The grip-width you use on barbell curls can make all the difference on your biceps growth. Using a standard, shoulder-width grip uses both the long (outer) head and short (inner) head of the biceps fairly evenly. The wider you go from there, the more short head emphasis and the narrower you go the more long head emphasis. So be sure to include in addition to shoulder-width grip curls, some wider-than-shoulder-width-grip curls to place extra stress on the short head, and close-grip curls to place extra stress on the long head. This will help to bring up the size of each biceps head, which together will create larger biceps