A recommendation from the folks in the commuting forum. It's pretty weird stuff, you put it on at night when you go to bed (it's a liquid roll-on). I don't know how it works, but it does...it was the only thing that would keep the 'pits dry when I was commuting in the summer. If you put it on your back, I think you might explode.
You should be wearing a wicking baselayer. That'll pull the sweat off of your back.