Replaced the compressor in a 1993 Toyota Paseo. The compressor works fine, but the serpentine belt keeps coming off ever since. It doesn’t come off right away, it takes about 1 days drive (80 miles). I’ve taken it back to the mechanic several times, but they can’t figure out what’s wrong. The last time they replaced some pulley, but the belt still came off. The problem disables the power steering and air conditioning (affected by the belt).
you have a damaged tensioner, or your mechanic isn’t putting enough tension on the belt. Have someone else fix it - he’s clearly inept at diagnosing this 
I agree with shadowfax. In addition to that check the alignment of the pulleys, it takes very little for the belt to walk off the pulley.