Out of everything I ever used over the years silicone spray worked the best.
However, if the belt looks ok and the tensioner seems to be working.. Try taking the belt off and cleaning all the pullies with a good carb cleaner or something of the such. Check the belt, both sides, to see if anything is on it or for any abnormalities.
Check and see if either edge of the belt has more wear than the other. I've seens pullies out of alignment very slightly that would end up causing any belt to start sqealing eventually.
HOWEVER, 9 times out of 10, I'd give the same advise I gavemy customers. Replace the belt AND/OR the tensioner.