OK people. Here's my patented technique to clean my wheels.
First, let me say that I have the stock 18 inch wheels on my Z.
1) Put your car on emergency brake.
2) Using a hydraulic jack, lift your car up the front until one of the tires can be rotated freely.
3) Wash the front of the wheel with your sponge.
4) (Here comes the trick) Take your sponge and place it inside the wheel and rotate the tire. The sponge will get cought up on the brake caliper. Keep rotating the tire and the sponge will take care of the situation. (This step works great if the wheels are not as dirty as a baby's diaper).
5) Repeat same procedure on all wheels (put supports on the front tires and disable the emergency brake when doing the rear wheels).
6) If you are not satisfied (which I doubt), use Meguiar's Hot Wheels Spray.