Sand around your rim, then wash it with warm, soapy water. Then do the same to the pads. Put it all back on, soak the rim with water, and ride around with the brake on for a bit. You should get bubbles all over the tyre, looks like foam? and some dirty water left on the rim. Just keep rinsing it off, and keep riding with the brake on. It should start getting louder and better each time you do it. Works for me every time. I haeve a smooth rim and Zoo pads. - Which get dirty and shitty before you even ride. So I pretty much have to clean it before each time I ride...