Valet Pro Bilberry wheel cleaner, dilute it depending on how dirty your wheels are (up to 1:10!), lasts a very long time this way. I use this weekly and don't even need to scrub my wheels, just rinse, spray on, leave for 5 mins and jet wash off.
http://www.cleanyourcar.co.uk/valet-pro-bilberry-wheel-cleaner/prod_499.html