Sergey Chikuyonok's Smashing Magazine article on PNG optimization is an oldie, but I just found myself returning to use it for maybe the 4th time since I originally read it. I don't do web graphics so frequently anymore, so I always return here to get reminded of things like the posterization layer in Photoshop. See also Sergey's JPEG Optimization article.

http://www.smashingmagazine.com/2009/07/15/clever-png-optimization-techniques/