When you wash the bike, wash the tires.
http://www.parktool.com/repair_help/BCB4.shtml
Park makes a tube/frame brush that's perfect for this, but . . . so's a $1.39 dishwashing brush that did it's time inside....
As for protectants and all that crap? Forget it. IMO, anything that'll "protect" your tires will degrade traction. Could be wrong, but....