All of the information here is good. I did the bearing mod, checked alignment, adjusted tension, etc. Most of these things worked...for a while. But eventually, the belt still squeaked and squealed.
So I fixed it...

I'm guessing any paste wax will work, but that's what I had.
I apply a liberal amount once or twice a year. I found that I usually need to do two coats each time. Slather some on, do a short ride, and repeat.
It hasn't squeaked this year, so maybe it holds up once you get enough worked into the belt. Or it could be that I no longer have a 20-mile freeway commute (more heat??). I'm not sure, maybe it will start up again this week.
I would still look into the other suggestions, but maybe this will help.
Good Luck.