The best way I found to learn to texture is look at other people's work. For metal, take this thread for instance:
http://www.game-artist.net/forums/fi...html#post68626
That's one of the best metal textures I have seen. Not overdone like some, simple and it gets the job done.
As far as good tutorials, check out the articles on texturing on this site. Yes, some do not involve metal, but you can take the skills learned in making any 'material' and transfer it over to others.
Some decent metal tutorials I have found are:
http://www.tutorio.com/tutorial/photoshop-metal-texture - skip the 'rust', dents and scratches and this is a decent starting point (needs a little modification though).
kirupa.com - Metal Texture
Grungy Metal Texture » Free Photoshop Tutorials - this one is a more painted metal look.