Edit (2013) - this is a bit outdated now, and was created in the days of painted-on lighting. Some of its still relevant though :]
This is a tutorial I made quite a while ago, but I think it'd be useful here. The whole tutorial's in the first screenshot, except for the first paragraph which had to be cropped out here due to image size restrictions.
"In this tutorial, i’ll try to explain how to make things look like certain materials, as occasionaly you’ll see a skin and think “hey, they’ve tried to do plastic but it looks like metal”, or stuff like that. I’ll be going over metal, plastic, chrome, and wood, since they’re the most common."
Let me know if you've got anything to add or that kind of thing, i've still got the psd for the picture so it's easy to change or add stuff. :]
Hopefully you’ve learnt a little from reading this tutorial, and I hope you can improve your technique as a result of it.
Thanks for reading!