1. Keep gauges clean. Dust gets on your car dash board, and can actually devalue your car if you are not careful. Every few weeks, take a soft damp cloth and carefully wipe the dust off the lenses of the various indicators on your dashboard. Be sure to wipe lightly, as hard wiping can lead to scratches, and this can make it difficult to read your indicators.
2. Get floor mats. This is especially important in the winter. If you live somewhere that is wet, snowy, or that uses salt on the roads, floor mats are a must. Get the rubber, waffle mats that can help protect your car carpet from salt, slush, mud, and other debris. They generally stay put, and are water proof, which means that all that stuff is not going to seep through and ruin your carpets. Just make sure you shake them out, wash them, vacuum them, or clean them when they need it, and replace them when they need that too. When cleaning them, use a high pressure hose, this will help dislodge dirt and debris.
3. Clean your interior regularly. Every time you wash your exterior, wash the interior as well. Get a bucket full of soapy water, and do a quick but thorough washing of vinyl and other surfaces. Also be sure to use a powerful vacuum to lift dirt and debris off carpets, seat upholstery etc. Soda, and other liquids, if spilled can corrode interior surfaces, so make sure that in addition to regular interior cleanings, that you immediately treat any spills that happen.
4. Keep rubber seals in good repair. Your windows and doors are sealed with rubber, which is what keeps drafts out, and what keeps your car interior quiet. Keep it in good repair by using a rubber protectant (such as Armor-All), and rubbing it down regularly. This keeps the rubber in good repair, and keeps the doors from sticking to it which can cause tearing, etc.
5. After cleaning, use a product to renew the fabric upholstery. A spray on Scotchgard, or other fabric protector will help keep upholstery clean, and make it easier to remove stains after spills, etc. Just be sure to test it before spraying it on to make sure it will not discolor your fabric.

There are several ways to protect your car interior, but regular cleanings, and some forethought can go a long way in keeping it in good repair, and making it last longer, and feel nicer. It is also going to make it much easier to sell should you wish to.