Just an FYI the whole WD40 thing... I've heard of this done before, and it doesn't have to be WD40, it can be any oil really. It's supposed to be applied to the REAR of the pad, not the pad surface. This will allow the rear of the pad to move slightly on the piston and not rub without lubricant. This stops squealing from this location. You need to remember that squealing isn't only caused by the pad vibrating/contacting with the rotor.
The other option to stop this and probably the more effective/long lasting is what 55ZED mentioned, you can get stuff called CRC Disk Brake Quiet which you apply to the rear of the pad and it acts like a glue which holds the pad to the piston so it doesn't flop around.