Squeaky brake pads can be a nuisance. The squeaking is caused by normal vibration in the pad that reaches an audible frequency. The sound can usually be remedied unless it is due to low quality brake pads that need to be replaced. Start with the easiest remedy to see if it solves the problem before attempting other solutions or replacing the brake pads altogether.
Inspect the brake pads to see if they are glazed. Brake pads often glaze over from normal use, the main cause being slow, gentle stops. If you brake hard and the squealing is not audible, glazing is likely the problem.
Scuff away the glazed surface using 1,000-grit fine sandpaper. Do not scuff hard--you are looking to remove only the superficial glaze layer.
Clean your rotors with brake cleaner and a rag, and then scuff them with 1,000-grit fine sandpaper. Use gentle motions to remove only the grime that has accumulated on the top layer. The sandpaper is fine enough that it will not scratch the rotors.
Unbolt the calipers and remove them. Some rotors are held on by pins, while others are bolted on. Check your owner's manual and use pliers or a wrench to remove them.
Remove the brake pads from the calipers.
Apply brake grease to the backs of the brake pads where they meet the calipers. Follow the directions on the can of brake grease. Do not grease the fronts of the brake pads because this compromises the gripping power of the brakes, making them less effective.
Reinstall the brake pads and calipers using the pins or bolts.
Never apply grease or any lubricant to the front of a brake pad. Brakes may squeak because they need to be replaced. If the pads are less than 1/8-inch thick, replace them.