Soak it with water. Use a soft brush to scrub it lightly with water and dishwashing soap (Dawn). Rinse thoroughly. Now, here is the important part - use your Shop Vac to pull out the water and dirt that remains in the carpet.
You can follow by brushing the nap up a little after using the vac.
My 2009 carpet still looks new.
Note: some recommend using a vinegar and and water for the wash. There are posts about this on the Bass Cat owner's board.
I fish because the voices in my head tell me to.