i've done the wd40 thing, but now i just use traction compound on the smooth side of the belt when it's on the car. i'll actually wet them prior to a run. seems to be the softest when they are wet.
something that hasn't been mentioned yet, is that the belts can shrink when soaked. infact, i've yet to find something that doesn't shrink them. while this can help a stretched belt remain useable, it also messes up the mesh between the belt and pullies. it isn't noticeable on the layshaft (small) pullies, but i can see it on the diff pullies. look closely with a loose belt. it will actually "bulge" above/below the pulley on the last few teeth due to improper mesh. i would think when running that it may not make a difference, or maybe the belt will stretch under tension enough to match. just something worth pointing out.