Some reasonably good advice and some silly suggestions there. I find the best way is to drive it into the sea (if you are lucky enough to live by the coast.) The salty water not only works as an anti-sceptic the sand particles will help to scour off the dried mud and when the water dries is leaves a nice white powder coating.
But please, do NOT put it on it's roof (as suggested above) to clean or inspect the underside, the loose change down the back of the seats will fall out and you'll wonder where all your endearing, little, mysterious rattles have gone when you are driving it.