Nice wide 5 spokes here

.. I clean the inside of the wheel with a microfiber cloth by hand. They still look mint!
Same here. The 18" have plenty of space for cleaning the inside. I just use a brush covered in a towel-like fabric and soapy water. Spray them, scrub/wipe a bit with the brush, rinse off.
Takes about 5 minutes per wheel, but they still look brand new.
Oh, and anytime I change the oil or rotate the tires I take all 4 wheels off, clean and polish them, then put them back on. Just the right amount of wax and the wheels will shine forever.
Then again, I'll be buying some black rims for my summer tires in the spring. Have a different plan for those =P