I use Simple Green. If you just driven around, wait until the wheels are cool before hosing them.
If your rims doesn't expose the rotors much, there are some vented disc plates that drop inside the wheels to minimize brake dust. Performance Products might still carry them.
The key is to keep your wheels waxed, that way the brake dust never gets baked on. Try using Clay Magic--works great. Also is great for getting a super smooth finish before waxing the paint.
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