Belt squealing driving me crazy!

I own a 95 Mitsubishi Eclipse GST with about 70 000 miles. I heard a terrible squealing noise from under the hood when I turned on the car. Soon after the squealing stayed even while I went through the gears. I had two belts changed to the right of the engine which I think were driving belts. It was quiet for about 2 weeks then started back up with the squealing again. I was at the shop today to get it fixed and all he did was tighten up the pulleys, and the noise stopped until I got home. Obviously this guy has no idea what the problem is and I really can’t stand to hear this noise any more. Please tell me what I can do to stop it.