How-to chrome your HTC HD2 battery cover
Here we have a great tutorial from blog.whitesites.com that will take you step by step through the process required for polishing your HTC HD2 battery cover into a perfect chrome mirrored finish.
In theory this would work with any battery cover that is made of metal, so even if your phone is not an HTC HD2, you should still be able to use this tutorial to do your phone ( as long as the part you are polishing is made of metal ). Its also possible to do this with Plastic but you have to be very gentle, and the course grit sand paper most likely will not be needed.