How do I get rid of a loud squeak in my rear brakes on my mountain bike?

Question

I have a mountain bike with frame mounted brakes. The rear brakes have always made a loud squeak even when I replace the pads.

Answer 1

Wipe the "rim" down since the first set of pads had one type of rubber, and the new ones had another.  The two different materials are working against each other.  DO NOT scrub the rim, use a cleaner solution.

Answer 2

Not sure about mountain bikes, but when my touring bike gets squealing brakes I scrub down the braking surface of the tire with acetone (lipstick remover :). It works!