Believe it or not, I've found SoftScrub to work well. If you have not heard of it - you'll be able to find it in the cleaning aisle of the market.
Apply a small bit of softscrub on an old rag, and gently work it circles, then rinse with water. But I'd use it sparingly, or dilute the softscrub with a bit of water. I used this on my nike tennis shoes. Oh, and if you have the choice, avoid the lemon scent - kind of weird.