I ran into the same problem three years ago and it seamed that no matter what I did, the squeak would return. You would tighten the belt and a few hundred miles later it would return and only with the AC running, right? When I purchased and installed the Goodyear "Gatorback" belts, I never had this problem again. The belts cost more then the average belt, but they work great!
http://www.goodyearbeltsandhose.com/cars/gatopoly.htmlWhen you tighten the belt, you'll have to turn the engine once or twice then re-check the tension again. What I do is tighten the belt, put the wheel back on without the plastic covers and test drive it with the AC on. Then I take the wheel back off, make final adjustments (if needed) and put it all back together again.
You need about 22 lbs of force between your Idler Pulley and your Crankshaft Pulley, but without a belt tension gauge this is hard to tell by feel. That's why I like to test drive it w/ AC on before I put everything back together. Remember that the AC belt is putting tension on your Crankshaft Pulley, so you do not want to over tighten this belt. You want the tention to be just right.