If your top is a cloth type material, you have to be very careful so you don't agitate and thus fray the fibers. Use a soft brush gently and you should be fine, just don't get to aggressive and scrub real hard while cleaning. After cleaning let the top completely dry and apply a protectant to help preserve the top material.
You can use Meguiar's M71 Canvas Cleaner, and M72 Canvas Protectant.
If your top is vinyl, then just clean it with a good cleaner, I like to use Meguiars All Purpose Cleaner dilluted to a 4:1. Then protect it with a good vinyl conditioner/protectant.
Let me know how this works out for you.