I just did biceps today. I focus way more on form now than I used to, but did manage to get back up to the weight I used to do on alternate dumbbells after avoiding them since I hurt my arm.
I start out with my palms facing inward. As I curl the weight up, I turn my wrist until my palm is facing upward. I try to make this one constant smooth motion.
This is only for alternate dumbbell curls. For preacher bench, I keep my wrist supinated for the entire rep.
Form is very important...but so is the weight used. I can benefit more now from curling a 45lbs dumbbell than I could have when that was the most I could lift. This is because I am now strong enough where a weight like that can be done for "slow reps" with way more mental focus on the muscle contracting.
This is something I never really understood before...you know, how some huge guy gets anything out of curling a 35lbs dumbbell.
I know now. What he got out of that 35lbs dumbbell is squeezing the muscle, increased volume and really focusing on the "feel" of that muscle working.
You can not get the same from it when this is the most you can lift.
So get stronger first.
I was going to post how much I am now curling but I'll just say I am back to where I used to be.