try to learn to feel the bis working. sqeeze at the top. when curling keep the elbows back and againist your ribs..one tip that I picked up for the Fit Show - was keeping your elbows back and dont let your elbows move in front of you. when your elbos move in front of you, the front delts get into it.
try pre exhause, supersets, drop sets and whatever to get your bis to grow.