What is the best way to add dirt/dust to AFV's?

Hey all

Just wondering what are some of your preferred methods for applying dirt or dust around the tracks, wheels and lower hull of the AFV. Airbrush? Dry pastels? Anything else? What colors seem to work best?

Thanks
Lance

I drybrushed my 72nd scale Tiger (see pic below…), but thats 72nd scale, so you can get away with a lot then.

Check this link out for some ideas:
http://www.missing-lynx.com/rare_world/rw05.htm

what I usually do is drybrush with earthtones and then apply ground pastel chalks to achieve the dirty / dusty look.