Make sure th ebrake is actually set up correct for a start. Clean the pads/ buy new ones if you must. Grind the rim or give it a good clean with fairy liquid an hot water. Use as little TPA as possible, so try and set the calipers straight and closeish to the rim. A 2p helps here with allignment. stick it between rim and pads, push pads straight an flat against it, align pads to the shape of the rim, tighten, remove coin an your set to go! Might have to bed the rim in now, just drag the brake for a few seconds. jobs a goodun!