An effective, inexpensive way is to go rent a "rug-doctor" or similar steam cleaner from the grocery store. Make sure to rent the extra "upholstery hose" attachment, which you will use exclusively. The rental should cost ~$25 or so.
Instead of buying the expensive (and heavily diluted) cleaners that they say to use, use plain Woolite laundry detergent, or the "low foaming" version (diluted in water per the bottle instructions for hand washing).
After you clean it, empty the machine and go over again with plain water to make sure all of the detergent is removed, then let the car dry out (a dehumidifier will help).
While you're at it, use it to clean up any stains in your house or apartment /ubbthreads/images/graemlins/dunno.gif