Oil them or give them a clean with a damp cloth
Clean the rims with rubbing alcohol or some other solvent and lightly sand the brake pads.
Oh GOD - do NOT use WD-40 as one person suggested. Brakes usually squeak because the pads are not "toed-in". The front of the pad should come in contact with the rim slightly before the rear of the pad. The way I do it is to find a matchbook - loosen the brake pads - slide the matchbook under the brake pads at the rear - then tighten. Before doing this, clean the rim of the wheel with alcohol & lightly sand the brake pads, thus removing any road grime or gunk.
