How do I weather my willys jeep to hint at general wear and tear?

Pick a brown/greyish colour and spray from a distance to mist over the vehicle. A nice touch can be achieved by masking the windscreen where the wipers move.
Hope this helps

Well what I did for the floor boards to giveit that scraped paint look was a technquie with saltwater. All there is to it is get yourself some salt from the cabinet and alittle water mix it up and get youself a brush and brush the area to be chipped. You need to paint the area a metal color first,let dry. Let the salt water dry then airbrush it with the base color. When dry scrape it off. All done.

Thank you for the ideas, but this is a 1/72 scale, so It will be kind of hard to do the salt water trick... I do not have an airbrush, so that is also out of the question. What is this dry brush technique that everyone talks about, and how is it done? (would like to hint at dirty tires)

Drybrushing is where you take a lighter shade than the item you are drybrushing and wipe off most of the paint on the brush onto a papertowel or sush, Then just drag the brush over the edges and it will leave a light coating of paint to simulate wear on the edges.

Another way to simulate dust is artists pastels. Get the chalk kind and use some sandpaper or the back of a hobby knife and grind it into a dust. Then take a clean dry brush and brush the dust onto your model. Make it heavier towrds the bottom where the vehichle would have been dirtier.

Hope that helps you out a bit.

http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=21624
there are some pictures of willys jeep, one is fairly dirty.
Just experiment some new techniques on an old model or something. FInd what works best for you

To add some light wear that shows the metal in areas like floor near the clutch and brakes pedal, floor at passenger side, rim of the hood, stepboards, etc. try rubbing a lead pencil against those areas. It will simulate those worn out areas well by showing some metallic luster in them.