Brakes Continue to Squeak

I have a 2005 Silverado 1500 (Z71). I started to hear a squeal from the front brakes at around 50,000 miles. I took it in to a shop I have used in the past for brakes in Oct 07. They did the typical brake job, turned the rotors and replaced the pads. When I picked up the truck, no squealing…that lasted about 10 minutes. I drove for a week hoping it would go away. (tried all the tricks, washing, hard breaking, etc.) I took it back to the shop. They said it could be bad pads, so they checked the rotors and replaced the pads….Same results. So I went back again and this time they turned the rotors again and replaced the pads. They asked I give it a month to break in…six weeks later same results. The brakes squeak when coming to a stop. It is especially bad at a slow stop. I took the truck in again. They once again turned the rotors and replaced with a different type of pad. This was a day ago. I didn't make it 10 miles from the shop when the brakes started squealing again. I called the shop and they said give it about 500 miles. I put over 800 miles on during the 6 weeks after the third set of pads. I have not had any problems with the shop in the past. They do good work and have been very cooperative in trying to fix the problem. I just don’t see the fix they are trying working. I am frustrated with trying the same fix for the problem. Any thoughts on what else should be checked? Any guidance is appreciated.
I did notice some squeaking this morning when backing out of the driveway. Didn't notice that previously