Grab state maps, or a Rand McNally US Atlas, for a general lay of the land.
Supplement with state DOT bike maps, if available.
Use www.bikeroutetoaster.com and play with the various displays, see what happens when you ask for bike routes versus car routes, look at the profiles, etc. (like bikely.com but a bit cooler)
If you have GPS, upload the route to your GPS.
If you like a cue sheet, print one out from it, or save/e-mail a pdf copy to yourself and check it with an internet cellphone from time to time, or take (legible) pictures of pieces of it with your digital camera, and view it on the screen from time to time.
And, like others have said, ask here. Put together a route you think might work, describe it or post a link, ask for comments and stand back. Sift the replies and work from there.