worn uniforms ?

Same way you do it on vehicles... Fading, Drybrushing , Pastels, and such with your "dirt" color...  Works best if you carry the ground colors up onto the legs and boots of your figures... It ties them into the terrain...  

I'll mix up a nice dirt color of ground pastel chalk for dust... Fading clothing to make it look older is about the same technique... Instead of pastels though, I use paint and dry brush lighter shades of the base-color...

Mansteins revenge wrote:

For faded uniforms, dilute your uniform color with white to get a bleached-out or faded look...scrubbing pastels on is a great way to dirty a uniform up in 35th scale...

IMHO white is too stark a lightening agent except maybe for mid grays. For greens, try adding some yellow or beige, with blacks, a flesh tone.