"Mr.Clean" Magic Erasers. They have many other brands available too. Get the kind which has the blue back. The blue works well on really tough stuff. After the white is gone(it eventually wears away) the blue gets destined for use in the bilge. I use lots of Dawn detergent.

Do not use tons of elbow grease. Lightly does it.

Used West Marines brand last week after using an entire bottle of Spray 9. Came right off with the West brand.

regular old Fantstik works great

"Vinyl Sauce" and magic eraser. The sauce stuff has a cleaner protectant and is designed for marine vinyl. http://boatbling.net/bbFrameset-1.htm

I just cleaned mine yesterday...................magic eraser's, nothin better............

soft scrub with bleach works great i use it all the time it was in one of the bigger boating mags a couple of years ago as a great cleaner ,i follow it up with a uv protectant

Warning, Bleach will dissolve the polyester thread, keep using it and plan on replacing or restitching, If you use it on the vinyl make sure to rinse the threads real good with water, We own a Boat Upholstery Shop, we have seen it all! Fantastic is recommended by G&T Products with SRT Finishes. Fountain and Baja uses their product. Each manufacture has it's own requirement.
Glad to see that everyone is using Protectant such as 303 or equal, best invention yet for durability and being able to keep it clean.
Another tip is to not let friends with precolored Tanning oil, it stains the vinyl as it has a petroleum base. Cover your seats in a towel

soft scrub with bleach

Krazy Klean works very well and for stubborn areas we use Bleach-White and have for years. Afterwards we condition the vinyl. Magic Erasers do work well, but if you have some very stubborn areas (because some crazy didn't remove their shoes ) we use a gray scotch brite (no reason to be aggressive) soaked in water and it has taken off very tough marks on the vinyl.