Stick the carpets in the washing machine (delicate wash, low spin), when they're done, dry them out on the radiator or just leave to dry naturally. When they're completely dry, get yourself a fabric sealant and spray all of the carpets liberally, leave overnight to dry and stick 'em back in the car.
The fabric seal coats the carpet fibres, this stops dirt from sticking and repels liquids. The seal should last for a few months, when you vacuum the carpets next time around, a lot less dirt is left on the carpet, if you're picky about a clean interior, this process is a must. I used Carplan Fabric Seal and Protect (available in Halfords), any fabric seal with Scotchgard will do the same thing.
CJ
