Keep the top clean using mild soap and water. Don't use any commercial cleaners or conditioners since they often dry out the material. Use a car cover, car port or garage religously. Verts are a little more maintenance but a lot more fun.
What about some kind of UV protectant? Maybe something that could darken it a little? My brother has a 99 Cobra that he was just asking me about this the other day. You would think that there must be something out there to protect it after you clean it...
I have been looking for a cleaner and protectant for my car and have mainly seen the mothers which has already been suggested here and aerospace 303. Have not used either so i can not give a review, but aerospace 303 is worth considering i believe. www.303products.com