I'm with Jonny on this one, wash with APC and a stuff brush (something like a nail brush) scrub until all the crud has come out then rinse with an open ended hose. Once it's fully dry, protect it with something like 303 fabric guard or Gtechniq i1 to keep it looking at its best and easy to maintain. Some people use another sealant which is used more in camping and such to help protect tents and other such things that aren't car related. Pretty sure it's called Fabsol or something like that...