I pressure wash, snow foam and combination of long handled wheel brush and wheel mitt in the front plastic arches (if needed, jet wash and snow foam brings most of the dirt off).
For the horrible carpety rear arches it's a bit more tricky - same process but as said you need a more rigid brush to loosen the stuff that sticks like glue to the carpet liner! Don't use your best wheel brush or mitt on this as you'll end up scratching your alloys. Keep something specifically just for that rear arch area.