I was thinking about the mention the other day about postmark collecting with use of a calendar, date, town- you get the idea. There is a lot of good information & links at the Post Mark Collector Clubs website:
http://www.postmarks.org/I have even considered purchasing the state post office lists available to members & nonmembers alike at a reasonable cost.
Another source is the Mobile Post Office Society site:
http://www.eskimo.com/~rkunz/mposhome.html They offer info on Railroad Post Offices, Highway Post Offices, Air Field Post Offices and just about any other mode of transportation used to carry the mail.
My postmark collection has taken on many forms: precancels, FDC, HPO, event cancels, circle date, spray on, cachets, first flight and probaly a few more I can't recall or name properly. I just started printed my pages to mount my Route 66 precancel collection (which is not complete) for display. I will be placing them in order by state groupings moving East to West along the highway. After I have done that I will move on to mount various types of covers & postcards. I hope to be able to transport the viewer to invision the places & people found along this historic highway in single frame exhibit of 19 pages. That in and of itself is a daunting task, I've set forth for myself.