Weathering.

melt nails and scratch and make holes in th surface to make it look like bullet damage and paint that part steel color and drybrush flat alluminum on parts that get a lot of wher, try rust colors etc and also put some light shades of the color of hte area on (different color for diferent colored area) and blend it in where paint might fade. yuse your immagination

It depends on what environment your model belongs in; you can always go for drybrushing a little aluminum paint on high-wear surfaces and joints, same for any mechanical details. If your model is one accustomed to wet or humid areas (like my RGM-79[G] master grade that I recently finished), give its surfaces a wash of chestnut or thinned down black ink, brushing heavily on top and letting off as you go down, to make a trickle-down effect of water and dirt.
If it's from a space environment, use the tip of a #11 blade and gouge out a few tiny spots to simulate micrometeor and space junk impacts. Then drybrush on a tiny bit of metallic paint in the gouges. Or draw on fine sandpaper with a soft pencil (2B, 4B or 6B should be fine) and rub it onto the model to simulate grime.
If it's from a desert setting get some yellow or brown pastels, grind them down with an X-acto and brush on and seal with the coat of your choice.
Happy modeling