Take your wheels off and spray the whole tire down with Aerospace 303 Protectant (After cleaning w/ a quality car soap and a normal sponge/brush of your choice).

The stuff is a little expensive, but it will help keep the tires clean and if you park in the sun it'll help keep the dry-rott/brown look away. Works great on plastic and rubber trim, too.

I like Meguiar's Tire Gel - It's purple and smells like grapes... I use it VERY sparingly though.. I don't like my tires looking like plastic... I may use it every other week, at most. Anymore than that and it slings everywhere and collects crap from EVERYTHING.