First off give the wheel a blast of water, using a pressure washer if you have one, to get off the excess crap.
Then smoother in Halfords alloy wheel cleaner and use a decent quality car brush to give em a good scrub, then rince off. By now they will be starting to look half decent.
Now it's time to get the pack of Wonder Wheels out and go over the entire wheel in detail with a healthy dose of the fluid using the pasty brush thing supplied. Mind your skin with this stuff though. Leave for a couple of minutes then rince off.
Finally get a bucket full of hot soapy water and an old sponge and just go over the entire wheel - this will stop any black 'streaking' when the wheel dries. Rince off and allow to dry naturally.
Using the above method each wheel takes me about 20-25 minutes to clean and they really do look as good as new when your finished and it all becomes worthwhile.
Just remember to take your wheels off every now and again if you're using Wonder Wheels beacuse while there's no doubt it works it also bonds yer wheels to the hub if used then left for a few months. Even better to take the wheels off when using it and allow to dry before refitting.
I know a lot of people who wouldn't touch it though...