next time it squeals, go to a garage with a high pressure airline and give the caliper area a good blast with compressed air (watch the old eyes !!). If sqeal goes it is probably just a build up of dust etc. If still there, consider having a look and filing the pad as discussed earlier.
NEVER blow brakes with an air line, the dust is highly toxic and carsonogenic, (most brake linings contain asbestos), it's NO NO, and it's a disciplinary (severe telling off at lest) in any reputable Garage.
Spray with a Brake Cleaning solution and then rinse with water, copper slip (as mentioned) on the back plates often solves the problem, and scrimping on fitting kits when new pads are fitted can also cause the problem, a quick fix, no I'm not joking, is to pour coke over the brakes.