I like to use pastels to weather the cockpit. In my case I use browns, grays, a little yellow or orange, and occasionally black. I shave off a mixture of these into a plastic cup and mix them up till I get a color that looks like dirt. Then I take an old brush and use it to put the pastel mix in areas that got dirty. The effect is pretty convincing, but if you plan to seal the cockpit with a clear matte coat, you will want to over do the effect initially as sealing it dulls it down quite a bit. Sometimes I just seal the paint underneath and leave the pastels on top unsealed. HTH
Ray
Here you can see the effect on a 1/72 UH-1B I built a long time ago. In this first shot you can see the effect is pretty subtle, but hopefully you can tell the area directly behind the pilot/copilot seats is much lett weathered than the area where the crew would walk most of the time.
Another shot from above that shows hopefully how I was trying to weather some spots more than others.
http://i176.photobucket.com/albums/w177/ultrasauros/Gladiator%20build/PICT2941.jpg