I buy this magical stuff at Walmart that works great. It's wheel cleaner in an orange spray bottle. I think it's called Eagle One? It works great - just spray it on, leave it for a few minutes, then spray the wheels down with water. Cleans the gunkiest wheels without scrubbing, no matter how long caked mud and and brake dust have been cooking in the hot sun.
For FUN (!!!), after that spray down your tires with Armorall tire foam. It's like spray painting your tires with a thick coat of fake christmas snow. Just discovered that stuff and it really is way too much fun to use

and it looks totally cool when you spray it on

. After the foam melts away, tires look shiny and brand new, even way down in the deep treads.
Good Luck, Jamie