Here's a handful of tutorials that should provide some useful guidelines:http://www.itchstudios.com/psg/main.php?id=ps55painthttp://www.itchstudios.com/psg/art_tut.htmhttp://www.gfxartist.com/features/tutorials/7731
Nothing really wrong with your approach, although I'd suggest painting the outlines first with thick, dark lines & a 'hard' brush (or do an entire silhouette, like Craig Mullins) if you plan to stick with a plain white background. That way, you'll retain edge definition when you start laying down the broad strokes and covering up most of the lines, instead of having fuzzy edges that'll need more work later on.
As for hinting at a metallic appearance while maintaining a dull texture, I'd suggest restricting any bright, sharp highlights to the corners, edges & junctions between flat planes. Any highlights on the planes themselves would be subtle & diffused, much like what you've already been doing.