I spent a whole day cleaning my car yesterday. Possibly not as bad as many would think because I find cleaning by car quite theraputic.  I start my hosing down the car, adding some wheel cleaning gel, then cleaning the car with 2 buckets of good quality car shampoo (NEVER washing up liquid as this contains salt), a rinse off using a power jet wash, then dressing the tyres, applying a layer of decent Carnauba (a high quality natural car wax), buffing to a fine shine and then tackling the soft top...

By now, the roof will have dried. As it's a fabric (rather than PVC) roof, I don't use a sponge or mitt to clean the roof because it leaves remnants of the cleaning item behind. Instead, I gently hose with a jet of cold water when rinsing the car off. Then, when dry, I use a soft clothes brush to gently remove hairs and debris etc (I also have a physchotic cat who makes a beeline for my car after I've cleaned it, leaving hair and dust everywhere!) taking care to ensure I don't apply too much pressure, particulary where the hood folds across the frame.

This alone now makes the roof look a lot better, but at this time of year, I also suggest part lowering your roof so you can check and clean the hood drain channels as these tend to get worse over winter with falling leaves and excess water backing up...

To finish the exterior of the roof (so it not only looks smart but also repels water adaquately well) I would suggest, possibly twice a year, applying a fabric protector like RAGGTOPP™ Fabric Protector. This not only 'sprays on' and 'wipes off' but it improves the look and colour of the top as well. It's a similar process to waterproofing a wax jacket or similar and given your car is probably worth a fair ammount more than your wax jacket, I think it's a good investment to undergo this process in the Autumn and Early Summer each year....There is a similar treatment available for PVC roofed cars.

Then, throughout winter, wash your car, and hood every week - religiously. The less time dirt has to work it's way into a hood, the less likelihood of damage and discolouration etc there will be...and with extra salt and other airborn substances and particles flying around in the colder months it's really a case of prevention, rather than cure.