The interior is the part of the car that you see and touch most often, so take good care of it!
General cleaning tips:
First, vacuum the entire interior. Before applying any of the detailing products, make sure to first test on an inconspicuous area for any color changes or damage. If so, do not use that product any further!
Some basic detailing supplies - armor all, carpet cleaner, brush, hand vacuum attachment
Vinyl or plastic: I recommend armor all or a wet microfiber towel. Over time, UV light will dry out and crack or curl the plastics, normally at the front of the dashboard.
Door bottom sills and really dirty plastics: I would use a paper towel since it will save wear and tear on your microfiber towels.
Glass: first make sure the car is out of sunlight and cool or else there will be lots of streaking. For cleaning, I recommend a wet terry cloth or a microfiber towel. Some professional detailers use newspapers to dry glass, but it might cause micro scratches. Some say that it won't cause scratches over time, but why not err on the side of caution? In addition, if you use newspapers to dry, you get ink all over your hands that may rub off onto already cleaned parts of the car. Again, just use a microfiber towel. If you have aftermarket tinted windows, do NOT use any ammonia containing cleaning products on glass, it will damage the tint. I say aftermarket tint because all automotive laminated glass has some light tint naturally built in. Since it is sandwiched inside the glass, you don't have to worry about scratching or damaging it. In general, I do not recommend windex for automotive use. The ammonia may also overspray and damage plastics and rubber seals. Two small drops of hand soap in a spray bottle filled with water will work fine. I personally just use a damp microfiber towel, then dry with a dry microfiber towel.
Seat care and stain removal
Seats: I actually prefer cloth seats because they grip better than smooth leather seats and are low maintenance. On a hot summer day, I sometimes have to put a cloth over black leather seats so I can actually sit down when I come back to the car. A windshield sun shade or tinted windows will help with that. If you have cloth seats, some Mr. Clean or soap diluted with water will do a great job on getting out that dirt. Carpet cleaner will also work once it's scrubbed in with a brush and then vacuumed out with a hand vacuum. Just make sure to let them air dry by leaving the windows open. Just vacuuming cloth seats is all the regular maintenance they really need unless you have stains.
Coffee stains: these are among the most difficult stains to get out. First, as soon as the coffee is spilled, try to absorb as much of the liquid as possible. If you are driving, make sure that you give full attention to driving first, as you don't want to cause an accident. Blot the stain, don't wipe because wiping spreads the stain. When you have a chance, mix a 50/50 mixture of vinegar and cold water and blot it into the stain. Use a clean towel to clean, then another clean towel to absorb the moisture. If the stain still remains, test a small inconspicuous area with hydrogen peroxide first, then try it on the stain. Do NOT use hot water on coffee. Coffee stains settle into the actual fibers and can come out again if you use hot water. The hot water "reactivates" the stain and can even make the stain set in further. If you want to get a very deep clean with a coffee stain, be prepared to remove the seat fabric entirely and wash it separately from the seat.
Here's an example of before and after coffee stains. You can only see the worst stains in the before picture. The after picture doesn't do justice to how clean the seat is.
Vinyl or leatherette seats are basically plastic, so use a vinyl seat safe cleaner and scrub away. Leatherette is not as breathable so it's almost always perforated on VW and Audi seats. If your Audi or VW seats are solid "leather" they're probably real leather.
Leather seats: these require the most maintenance out of all other seat styles and are often the most abused. When you clean leather, keep in mind that it is basically cow skin, and requires to be cleaned gently. Dirt and oils from your skin soak into the leather and damage it over time. If you look at your seat and the leather is shiny on the seat and dull on the back of the headrest or parts that you normally don't touch, it because the contact areas are soaked with oils from your skin. Cleaned leather should have a dull sheen to it.
I do not recommend an all-in-one cleaner for leather. You can't thoroughly clean and moisturize your hands in one step, so why would you try to do the same with a cow skin? Use separate cleaners and conditioners. Clean the leather one section at a time with a soft brush. Then rub in conditioner with a sponge. Wipe off any excess with a microfiber cloth. It is best to clean and condition when the leather is warm and can absorb more conditioner, so try closing the windows and letting the interior of the car heat up before cleaning.
If you just got a used car and the leather seats are hard and cracked, you may be wondering what you can do to restore them to like new condition. They can be softened to a degree, but once the leather is cracked, it is permanently damaged. There are glue-like products that can close the crack, but they need to be painted over and may crack again. If my seats had small cracks, I would immediately clean and condition the seats to stop any more damage, then begin a series of softening treatments. There is an oil called leatherique that supposedly can make hard seats soft again if the oil is allowed to soak into warm leather. I have not tried it, but many people swear by it. Topoftheline auto detailing supply also sells oil reconditioner for leather seats.