Well usually a vacuum would suffice but if mud was involved I'll suggest dumping it into the washing machine. Just make sure you tie up the loose plastic ends so they don't break. Also don't mix it with clothes! A seperate load would be preferble for obvious reasons. Once you're done it's best not to use a dryer but let it air-dry.
I use a Canon 200EG as my backpack btw. But so far it hasn't got dirty enough to warrant a cleaning.
I usually clean my camera bags with a damp cloth. So far there is no need to wash them yet. The dirtiest they get is just from dusts on the bag exterior & a damp cloth works just fine in removing them.
it is batter to send it for dry cleaner becuase if you use soak it in water, and u din dry the bag throughly, dampness will present in the fabric of the bag and cost your equipment to rot and growth fungi.