How much of bicep exercise?

I started working out more than a year ago but am still not very happy with my biceps .... I used to do 3 exercises with 8 reps each until very recently when alot of ppl told me that I was over training my biceps. I always train my biceps along with my back. SO now I alternate between two bicep exercises (8 reps each) :

1. Barbell curls
2. Preacher curls

OR

1. Inclined dumbell curls
2. E-Z bar curls

I try variation, I lift heavy and I pay strict attention to form. Hell what am I doing wrong??????? please advice
 
Try concentration curls with isometric holds on your last 2 sets of your last bicep exercise (concen. curls)
 
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