If you have a standing preacher at your gym try doing curls by leaning your torsoe over the angled side... your arms will be hanging vertically down the flat side of the padding. I find that the tension if entirely on my bi's.
Doing preachers with my arms on the angled pad seemed to put all the stress of the movement on my joints and forearms rather than my bis. Once your strength increases with this movement, other exercises like barbell curls will increase as well.
R.