One way to really improve the appearance of you interior is to clean your plastic door sill panels. It might not seem like a big deal, but it can really take some age off of the look of your interior.
I usually do this every few months, right before a car wash.
To remove the trim panels from most vehicles you just get your finger tips under the edges of the panel and pull straight up and it will pop off.
Take 'em outside and spray 'em with a good cleaner. I use Griot's Garage Rubber Cleaner, but something like Simple Green will work too. Then use a stiff brush and scrub 'em all over. Let it sit for a few minutes. (This is when I get my car wash stuff out and get ready to wash my truck.) Then just hose 'em off. A second application might be needed. Then let 'em air dry or wipe em off with a towel.
If you have some scuffs that won't wash off, try a damp Mr. Clean Magic Eraser, or Goo Gone and a rag.
To re-install them, just line the peice up where it goes and hammer it down with your fist. You should hear the clips snap back into place.
Give it a try, it's worth the extra 10 minutes.