I tend to give it an overbrush of what I suppose is roughly chainmail (I use Reaper Master Series paints... it's called Honed Steel from that line) and use a little black ink thinned with water to paint into the recesses.
On my Sons of Horus, I paint it gold to match the gold banding and whatnot on the rest of the model, and touch the recesses with some watered down chestnut ink instead of black ink.
Also, you might try blacklining the join between the mask and the flesh to add a bit more depth. That takes a steady hand, though, so if you're not confident with it, it might be easier to just highlight it up a bit to add some artificial depth.
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