My usual technique is to wash with Nuln oil and then once the wash is dry, lightly drybrush over a bronze colour (Brass scorpion or Sycorax bronze are my usual choices). With the drybrushing use a soft, large brush and ensure the brush strokes are all in the same direction to get a "brushed steel" effect into the flat surfaces.
The raised areas become a dirty bronze colour, whilst the recesses are dark silver.
I've added a couple of shots behind a spoiler mark to show the effect on my Warboss in mega armour.
The shoulder pad in this shot is a very light drybrush, whilst the axe is based from Warplock Bronze
In this shot, you can see the brushed effect. The biggest secret to this one is an old, worn brush that doesn't give a smooth coat.
Finally the rear armour in this shot shows how the technique picks out the raised surfaces with bronze, but leaves the underlying areas in a more silvery colour. This helps add depth very quickly to a complicated surface without having to spend hours painting the individual colours in.