A belt can squeal for 3 reasons.
(1) Pulley misalignment.
If even one of the pulleys is slightly misaligned, the belt will constantly correct by slipping sideways on a pulley. This results in a constant chirp or squeal. After a whle the belt may start to show some edge wear if the misalignment is severe enough. Often you can see where the misalignment is occurring by closely watching the belt as it is running. Check to see if the power steering pump pulley is pressed on flush with the end of the shaft; PS pulleys can start to walk off after many thousands of miles. If a component such as an alternator was recently replaced, that may be related; many rebuilt parts do not have the pulleys correctly aligned (especially if from discount parts stores like Autozone, PepBoys, or Advance). If your engine bracket design is of the type used on late 80's/ early 90's GM trucks where there is a large sheet metal bracket that covers much of the front of the engine and to which all of the belt driven components are attached, check carefully for cracks or loose mounting bolts that hold the bracket to the front of the engine (common failure). Check to make sure the harmonic balancer is not coming apart or walking off of the crankshaft. If any pulleys have been replaed, count the number of grooves to make sure the pulley is correct and matches the belt ribs. I have seen instances where an alternator, for example, has come with a 7 groove pulley to fit a 6 groove belt vehicle.
(2) Incorrect Tension.
It is extremely important that the belt be properly tensioned, or it will slip under load (such as when the engine is started). If the belt tensioner is original, it should be replaced as teh internal spring does become fatigued with age and use. I would recommend using a GM tensioner; many discount parts store tensioners have given us problems over the years because of poor manufacturing tolerances such as incorrect spring tension, bearing play, and pulley misalignment.
(3) Belt contamination.
If any foreign substance such as antifreeze, oil, or power steering fluid should contact the belt surface it will be absorbed and cause a change in frictional properties at that point in the belt surface. This also applies to greasy fingerprints, or any kind of "belt treatment" spray. IF a belt should become contaminated, it will never run quietly and must be replaced, making sure it is clean and dry. Check for water pump leakage, PS pump leakage, or for any other kind of substthat could be causing contamination of the belt surfaces.
Also, I have found that some brands of belts are very stiff, and tend to slip when cold. I have personally run into this with Kelly brand belts and the cheap line belts sold at Autozone and NAPA stores. If you are installing one of these brands, you may want to try using a different premium line belt from a different supplier to see if it resolves the problem.
I hope this is helpful!