My buddy has a convertible, he said he uses something called 303 cleaner on his top, suppose to be real good. Here's a site that sells it.
303 Aerospace Cleaner,303 Aerospace Protectant, 303 products As far as cleaning, I would expect that you would clean the top like any other part of your car, so no, just washing wont take away all the dirt and grime that might be sitting on it, especially if you don't garage your car.