I have a heap of respect for the icon designers out there. These people start with a blank page and after several hours they end with a tiny graphic that communicates a heck of a lot of information. It takes a lot of talent and clever thinking to simplify an idea or concept into an understandable graphic.
Von. R. Glitschka created a step by step walk through detailing the process he uses to design an icon. The zipped download (1.8 Mb) has a set of images from each step with related notes in a separate text file. You can learn a lot about design by studying the processes of others.
More about the Tango icon library can be found at http://tango.freedesktop.org/Tango_Desktop_Project