make sure the braking surface of the rims are clean. I'll take some rubbing alcohol and wipe them down to get any oil/dirt,ect off of them. If they are really dirty take a rough sponge and clean them then use the alcohol. Some people use brillo pads but I don't like the idea of possibly scuffing/scratching the surface up with those. Also check your pads every so often and if they're glazed take a piece of sandpaper and remove the glaze from them.