Wax them, or get wheel cleaner specific for painted wheels. I can't remember the stuff I used on my X's when they were painted, but it worked miracles (and got the axle grease from the sweeet single slammer axle boot I had that had been torn FOREVER.
Next time I see the brand of it I'll post it. It comes in a white spray bottle. Each bottle is enough to do a set of wheels thoroughly about twice.