Nathan,
For metal, I use the "Martin Method". Brown, way beyond normal, no carding between coats, till you get a very rough, pitted result. Sand it down till all that's left is the pitting. The cold blue, knocking off the edges with scotchbrite pad and oil, till you get that nice French gray sort of color. Looks like a well taken care of old gun look.
For wood, after sufficient stain and finish, I use a combo off the same finish, with some lamp black and Burt sienna, in the low areas. Blended so it looks natural. When aged to your liking, another coat or two of the original finish to seal it all. Almost forgot, when staining, feather out the stain in "wear" areas.
Hope that helps.
Below is an example that is lightly aged.