There are a number of different "systems" that you can use - I'm in the process of trying all of them as I need to.
Renovo: consists of a cleaner, restorer/reviver, and waterproofer. My review of Renovo - overpriced. The cleaner worked alright. The restorer works very well, making the top very black again. The UltraProofer repels water in a light mist, but once it's wet - it's wet.
303: There is 303 Convertible Top and Fabric cleaner, and 303 Fabric Protector. I use both on all the Sunbrella canvas on my boat and love it. Will be doing the Saab's top next week with these products. A little pricey, but not anywhere near the Renovo!
Raggtopp: Haven't used these yet, but hear fairly good reviews on it.
If I were you, I would get the 303 products - just my 2 cents...