Did you "toe-in" the brake pads?
If not: Adjust the pads so that the back edge has about .5 to 1 mm clearance when the front edge touches.
I've found the easiest way to do this is to loosen the pad and then hold it tight to the rim with the brake lever with a piece of brake/derailleur cable or a small zip tie under the back edge of the pad. Spmetime it helps to squeeze and release a few times to let the pad self adjust. When it looks good tighten it up!