I use a towel in the same way Scott uses a baby blanket.
I also believe there is a soft top protectant sold by Porsche. Many swear by it.
Here it is:
http://www.properautocare.com/porteqcabtop.html
Then there's a slew of other items depending on what you're looking to clean or preserve:
http://www.properautocare.com/besprodforcl.html
This company recommends "Raggtopp" protectant stating that it's recommended by Haartz and other top manufacturers.
Worth noting:
".... Both the Haartz Corporation and Rode & Schwalenberg Gmbh. (Germany) recommend only Wolfstein's Raggtopp Protectant for treating fabric convertible tops. Haartz and R & S supply 98% of the convertible top material used by the world's automakers. (There is a small supplier in Japan that supplies topping fabric to some of Japan's automakers.)
Over three years of product research and testing has gone into this product. Raggtopp Protectant micro-bonds to the convertible top material, renewing and extending the life of fabric tops. Water repellency renews after each treatment once the top is completely dry. Raggtopp Protectant contains no silicone, freon or CFC's. It is ozone friendly and meets California air quality standards. This is the only product for protecting fabric convertible tops that passed all of Haartz Corporation's extensive tests and is the only product endorsed by Haartz and R & S. (Porsche Tequipment Cabrio Top Protectant and 303 High Tech Fabric Guard did not pass all of Haartz Corporation's tests and are not approved by Haartz or R & S.)"