It's a toss up between two techniques from ScaleModelMedic. Finding these compelled me to start modeling again. Just really amazing stuff that anyone can implement and totally change the models impact.

Dust and dried mud

http://www.youtube.com/user/ScaleModelMedic#p/u/14/nx1e6ooh8PA

and weathering with oils.

http://www.youtube.com/user/ScaleModelMedic#p/u/17/kOLcQVBa87E