My commuter bike announced every stop to the entire street. Here is what finally fixed it,
in the order I should have tried things.
Clean the rim. Isopropyl alcohol and a clean rag. Road grime and a little oil
are behind most of the noise.
Check the pads. If they are glazed, rub them on fine sandpaper. If the groove
is gone, replace them.
Toe-in. The front of each pad should touch the rim slightly before the back.
A folded piece of cardboard behind the pad while tightening gives about the right angle.
Tighten everything. A loose brake arm will squeal no matter what else you do.
Step three was the one that worked for me. It took four tries to get both sides even, but the
bike now stops in polite silence.