I recently saw Andrew Price's tutorial on how to do realistic texturing. At the end of the tutorial he said something along the lines of needing to just "add scratches" to increase the realism. How do you simply add scratches? I understand that there are probably multiple methods, and I am interested in hearing as many as possible. Thank you.
I would just find an appropriate image texture of scratch marks, and then use that as a factor in mixing your diffuse and glossy shaders. You could also use the same texture as a subtle bump map factor.
In the screenshot below, I'm combining the scratch texture with a fresnel effect before using that output as a mix factor between the diffuse and glossy shader nodes.
You can find lots of scratch textures at http://www.cgtextures.com