I have a garage full of every single detailing product that's ever been discussed.
In every category, leather, wheel cleaning, clay, canvas, tire dressing and that's before we get into the porter cable and that whole universe of products. Then I have my special mountain bike/road cleaning products which have its own cleaners and brushes. I have large bins of job-specific towels. I also recently starting using a foam gun as well.
All that being said... The best thing for your paint, is to touch it as little as possible.
That 991 that was dirty might look horrible but as long as the paint is waxed and sealed, is better off than being tinkered by someone who doesn't have the know how. The biggest culprit are using the wrong types of towels for buffing and drying. Those microfiber towels sold at auto stores are especially harmful. Those are basically re-labeled janitorial towels used to clean dirty floors. Using these on a well maintained car is like opting for a sledgehammer when rubber spatula will do the job.
This is the best thing to do: Use a foam gun (check
autogeek.net ~$60) and spray in front of the chenille wash mitt so that anything you are touching is well lubricated and slick. Keep a few of those clean mitts, don't use one to clean the whole car because aferall those mitts are inexpensive. Dragging road sediments from a dirty mitt from one side of the car to the other is unecessarily harmful. Use a clean mitt for the left side, another clean one for the right side and a third for front and rear. For the lower portions use the least dirty mitt. Then for drying, spray down the car with a quick detailer to lubricate the drying process if you're using a waffle weave towel. One long pull of the towel across the surface. But the best thing is to simply drive the car to dry it or use a blower. If you're going to re-apply a spray wax booster use a long nap towel like the Griot's green or blue towels. NO PRICKLY microfiber for buffing. DON'T use a waterblade as it goes against the entire concept of increasing lubrication on touched surfaces. When applying waxes or polishing wax cleaners or sealants, spray the surface with a quick detailer spray (any brand will do) before the applicator sponge hits the surface and after the product hazes over spray the surface again. There should always be a thin film of lubricant when pressure is being applied to the paint. And limit these waxing sessions to as little as possible as it is very invasive to the paint, you're better off simply re-applying the spray wax booster of your choice more frequently. Right now after washing I'm using Griot's spray wax. A little while later after its dried I use Finish Kare #425 spray to seal it up, it has a very very slick teflon like finish which makes it easier to clean the next time around. I buy it by the gallon as it works on all metallic, glass and plastic surfaces.
As far interior and wheels, knock yourself out and clean as often as you like.