Hi Dave,
I use Autoglym tyre dressing, sprayed onto car tyres for shows - it's a milky liquid in a trigger spray bottle. I don't spray it on bike tyres directly, for fear of it going onto the tread but I spray it onto a cloth and wipe it around the sides of the tyres. Just leave after application, no rinsing or wiping off and it's supposed to be beneficial to the tyre compounds (but they would say that, wouldn't they?). Does go a long way though and the finish is akin to new rubber - not like those on a cheap used car lot!
Ian