I find the best way is to soak it with a good alloy wheel cleaner (I've been using the Wonder Wheels Colour Active one for quite a few years now as its widely available and decent enough for the money. There may be better ones around now) using a brush to agitate it into the difficult areas then leaving it for a few minutes before rinsing. It won't get the most stubborn brake dust that has pitted into the paint but it will soften it. Sometimes a second application will get it out but if it has really dug itself into the paint it'll likely need to be sanded back with fine wet and dry before polishing it back out.
