you could try a belt dressing, which would only be temporary...... best option would be to replace it.... what year is your ride? how much mileage? are you sure its the belt itself, and not a pulley on the idler, or alternator?
I read on here about some guy who cleaned his splines with a wet paintbrush while the engine was running and that took care of the noise. He'd changed belts, pulleys, tensioners, alternator,etc. It turned out to be the auto car wash blowing crap onto his belt which made it squeak. But if you have 5 trillion miles on it, then you should probably change your belt.
I agree with Matt- it could be lots of things, but without knowing mileage etc.. Ford has had issues with tensioners in the past, but as Fatal says, the belt could be dirty as well. If you have 90K on it, I'd probably change the belt. Goodyear Gatorback is a great belt.