This link has been going around Facebook especially in the past couple of weeks. Though I don't live in the country, I think this is a great way to record where I've been and for approximately how long.
My key to the colouring:
Red: passed through (on a bus or train or airport layover)
Orange: short stay (a few days)
Blue: long stay (a week or more)
Green: Several weeks.
I realised after making it that Colorado should be blue, as I have spent more than a week there in total. But I'm not going back to start over and recolour all the other states too, so there we have it.
Make your own at http://www.defocus.net/visitedstates/generate.html.