There's an helpful article on putting together a home office in the March 2008 issue of Log Homes Illustrated. She suggests making it a dedicated space [there are too many guest room/office combos], plenty of space (including for lounging), keeping it neat (files for papers, etc.), and being sure it has a door.
Another interesting article in the same issue is about having a home office "off the grid." (I always enjoy Rex A. Ewing's "off the grid" articles.) He discusses the power usage of computers, monitors, printers, and scanners; the complexity of battery storage; and the difficulties of Internet connections when you're that far off the grid. Very illuminating.