Soapy water and a soft brush. I personally have been using the "do-it-yourself" coin-op wash since I bought my car last Jan. I pre-soak it with the soapy water if there's any bird "surprises" on it while I scrub my wheels/tires, then attack it with the soapy brush. along with the rest of the car. I give it a good steady dousing with the Clear Coat Protectant as well - trying to keep it evenly applied and rinse with the Spot-Free rinse and it looks like new.
There is a product which I haven't yet tried but I've hear a lot of BMW owners swear by it, called "Ragg Topp" which is a spray-on treatment (like Scotch Guard I assume) and will try to get some one of these days and report back.
As I rarely put the top up these days, I hardly ever see it, but it also has dirty creases from being folded up too long, so I need to wash the car about once a month and that keeps them at bay - plus the heat from the sun warming up the wet top seems to stretch it back to showroom condition when I'm done.
Hope this helps - enjoy the ride!
Jeff