I've also recently discovered belt conditioner. I had a problem with a belt in october and it had been very cold out, i sprayed it with this stuff because i was at school and it stopped the squeaking and slipping
just so you know, this is only a band-aid. it will start slipping again (actually it probably still is), and most likely, by that point it will need replaced. what that stuff does, is lube the belt, makes it sticky, the combo makes it stop squealing, usually temporary. better to just tighten the belt. also, since it usually stops the squeal, but not the slip, it doesnt let you know untill the belt breaks, leaving you stranded.
just my $.02
aaron