I hate so say this, but, you might be having a more serious problem. first check the tension on the belts. find the longest section between pulleys and move the belt in and out, the total deflection should be around 3/4 of an inch. If this is correct, then the problem could be that the compressor is going bad. If the bearings inside the compressor are beginning to seize, then there will be too much resistance causing the belt to slip in the pulley. Since this pulley will become glazed, as I mentioned before, it is really important to clean it. It might require the use of sand paper. 80 grit won't hurt it to help gain grip. Another possibility that is much less expensive, could be that the belts are the correct length, but, not the correct width. This is some what unlikely. To check, simply look at the top of the pulley and make sure the belt is riding about 1/16 to 1/8 of an inch above the pulley edge. If it riding too lowthen the sides are probably not in "pressure" contact and it is riding on the bottom, or, if too much is out of the pulley, you have the same issue not enough surface area is in contact between the belt and the pulley. A warning though is not to overtighten the belts as this will cause premature bearing wear in all the components, water pumps especially.