As someone who lives in an apartment and doesn't have access to a hose/water, I'm in the same boat.
Amazon.com: Griot's Garage 11066 Spray-On Car Wash - 1 Gallon: Automotive
I've been using this waterless car wash and it's pretty awesome. If the car is really dirty, you are still going to have to use water to get the big stuff off, but this is great to use to keep the car clean without wasting water.