Concentration curls with full twist (suppination?). Rotate the pinky towards the outside part of the delt at the top of the movement and squeeze it. Also might want to try double cable curls with a high anchor point following the same form as the conc. curls. Genetics also plays a part in it.

Don't forget the grandaddy either. Heavy barbell curls with good form for most of the reps. Cheat only on the last couple of reps and accentuate the negative.