I don't paint armor, but I have painted machine guns.
My favorite technique has been to paint it gunmetal, followed by a flat/dull coat and then drybrush the hightlights with silver. Depending on the scale, I'll sometimes also rub it down with a pencil. The graphite looks like a worn-down bluing (just do it before the drubrushing).
I don't have any pictures of previous stuff, but it's always looked pretty good to me.
-Fred