Here is my gospel on avoiding fine scratches, swirl marks, oxidation, and the like. My advice is different from most here, in that I emphasize prevention and "first do no harm."
Step zero is to wait at least 3 months after your car's build date (look inside doorway) before doing anything but a gentle wash and dry.
You will get tons of opinions about waxes to use. Choice of wax is less important than the other points on my list. I happen to prefer synthetic (polymer) and avoid Carnuba.
One fellow who has tested waxes on his dark cars at length is Andre, an S6 owner from South Africa, same color as my car (below). Here is his advice: Andre on Sonax/Zaino Polish-waxes
. Also look elsewhere on his site for updates. He knows what he is talking about!
I use a professional shop, here in Silicon Valley, called AJ's in San Jose. They use all the right materials and methods I list here.
First priority: protect with wax. Wax the car every 3 months, or sooner when it no longer feels smooth after a wash or beads water.
Minimize touching of the paint. Scratches and swirls are primarily in the clear coat, and can't be fully repaired with polishes and waxes. So the best approach is to avoid doing anything! Seriously.
Only use a car duster when the car is clean and has only lint-like dust. If the car has been rained on or even has a bit of mist or dew on (except when totally clean), or if there are any tiny spots that are sticky or slimy (tree droplets, soot from airplanes), or if the dust is gritty (soil dust, sand) -- then do not dust it until after the next wash. I usually only dust for 2-3 days after a wash in good weather; less in damp weather.
Only wash it (yourself or by a pro) using specific materials and techniques. Materials: Microfiber wash mits (try http://classic-accessories.com/exteriorcare.html
), microfiber towels (two large is enough in a dry climate; mine are from griotsgarage.com), very soft wheel brush (OXO makes great ones, works for dusting dry between washes too), high quality car wash soap (I use Audi's own, by One Grand), and so on.
If you get bird spots between washes, clean them off right away with distilled water and a microfiber cloth (like griotsgarage.com cloths for removing wax). Typically, your car is dirty, so you don't want to wipe much. Spray on the cloth and blot. Only use a detailing spray if the car is clean, because the overspray requires more rubbing.
Wash carefully: No high-pressure spray, especially on a dirty car. No hard water (purified, decalcified, especially for final rinse). Wipe along the length of the car to minimize scratches (don't go in a circle). Do wheels first with a non-acid cleaner (P21S is great). Then wash the car once, and to make sure you have no water spots, do one more quick wash before final rinsing. Rinse the wash mits with a hose before dipping again in the wash water. I use 2 mits so that when one looks/feels a bit dirty, I swap to the other, then rinse, then rinse both mits, and continue.
Make sure you have actually washed the car clean before finishing (draw fingertips across paint).
If necessary, use clay to remove deposits (works well on windows too). Read up on clay at Griotsgarage.com or elsewhere. I think you should limit this to just before a wax job. If the grime is stubborn, use a gentle solvent or clay as appropriate, and rewax.
After the final rinse, do a gentle flowing rinse so that the water sheets off (not a spray, but a smooth flow without a spray nozzle). then dry by dragging the towels across the roof, the hood, the trunk, then the sides, and so on, with increasing attention to detail. To prevent dribbling and spotting, I use a blower to force water out of lamps, grilles, handles, and trim, and then dry each area again as I go around. Keep dyring around and around iteratively to prevent spotting.
Most car washes are to be avoided. Problems: Wheels get scratched by mechanism. High-pressure sprays can scratch paint. Dirty mits cause scratches. Terry towels remove wax and can cause scratches.
Water spotting is a terrible enemy, the bane of dark cars in particular! Park away from lawns where there might be sprinklers. Even washing can cause spots.... Municipal water is often very calcified or acidic (I use two "Calcium Inhibiter Filters" from Arizona Mist/Orbit Irrigation, 800-488-6156-1-4, screwed onto the hose for in-line calcium removal). Even rain water is acidic in some industrial areas.
Wash as soon as possible after any spotting, like the same day, because it can etch permanently. Remove water spots carefully, first try normal washing, and if that fails, with special products specifically for this purpose, or 50/50% vinegar. Polish only if necessary at this point, and minimally. Either way, you may need to wax again.
Never place a car cover on a dirty car. Never drag the cover across the paint any more than possible. Use the lightest weight, most supple fabric that meets your weather and breathability needs.
Minimize polishes, glazes, and even washing unnecessarily, to avoid the fine scratches, the ones that pick up the light and look like fine parallel lines, concentric circles, or spider webs.
Swirl marks (foggy streaks that seem to have a 3-D quality) have never been a problem for me. They come from misuse of waxes, polishes, glazes, and even poor washing. Over-working the car (as many people on this forum probably do and recommend to others) is to be avoided!
Park far from other cars and walk! Park away from trees that can drop sap droplets or other detritus. (I had a back injury and so used a disabled placard, and that helped avoid door dings or people brushing against your dirty car or setting things on the trunk. The car looks great!) So, get some exercise and walk.
Get a Stongard clear-bra for the front. Audi paint on the bumper chips like crazy. Use a quality installer, don't cheap out on this. Quality materials should be used (3M for example).
See my FAQ
below for a growing car care list of links
and other subjects.