I've tried sanding them down just a bit, cleaning the brake surface on the wheels, and this does stop them from squealing for a day or two, but it always comes back. I'm using Kool-stop pads if it matters.
There are a few causes for brake 'squealing':
- New pads. After a bit of wear (or sandpapering), the squealing will stop.
- Misalignment. As some others have mentioned, misalignment may cause squealing (although it would be reduced with some wear). Check out Sheldon Brown's tips for alignment suggestions.
- Oil or water on the rim. Either of these substances, even in small portions, combined with dirt or debris on the rim can create squealing. Wipe the rims thoroughly on the braking surface, as well as the pads, with a dry cloth.
- Incompatible rims/pads. No one else has mentioned this, but this is often the most common cause. Rims are generally made from one of two materials: steel or alloy. (You can tell the difference by seeing if a magnet will attract (steel) or not (alloy)). Incidentally, brake pads are made of specific compounds to work on one or the other. Often, using pads designed for alloy rims (like most Kool-Stops) will squeal if used on steel (which I'm guessing you have).
Brakes have to be toed in. What that means is that the front of the brake pad has to touch the rim before the back of the brake pad. It does not have to be an extreme variance but it will make a world of difference when properly adjusted.
Hopefully you have aluminum rims. I have had the misfortune of working on some bikes with steel rims from which I could not remove the squealing.