) Use CRC Stop Squeak (pink adhesive goo) available from Repco and most autopart places. This rubbery adhesive goes on the back of the pad (and if you still have them the anti-rattle shims) and in 95% of cases stop the audible squeal from pad vibration. I use it all the time on all 5 of my cars. See Brake System - CRC Disc Brake Quiet (4 oz.)
b) If this doesn't work you can cut a radial groove (same direction as if you had spokes in the wheel) in the pad with a hacksaw in the middle of the pad. Cut down to within about 2mm of the backing plate. You can have two radial cuts if you use a hacksaw or one if you use an angle grinder. Some good pads already have a radial groove or cut in the middle of the pad for this reason. this both helps gets rid of squeak causing dust and changes the frequency of that sound.
c) Chamfer the leading edge of the pads (the short side) so the angle of incidence with disk is around 45 degrees instead of 90 degrees. Just grind or cut or chisel the corner off the leading edges (just about 5 mm usually). This changes the frequency of the sound.
d) Sand the disks lightly.
These fixes will not inhibit the performance of your brakes and good pads will often come with the radial cuts and or chamfers.
Also check that the disks are not worn to the extent there is a lip or high spot running around the outer edge of the disk (where the pad doesn't normally may contact). This will cause a squeal as the pads hit this high spot on breaking and vibrate; to fix this the disks will need to be machined or replaced.