Although safiri was my route building mentor, our methods and tools are not exactly the same. We both start, however, with the same goal---to find the most curves, the most hills, the most stream crossings, and the most rugged roads we can find in the general area we plan to explore.
Sometimes I know a general area or direction I want to explore, and then look for roads in that area. Sometimes I don't know where I want to explore, so I start looking at maps and satellite images looking for possible target areas based on lakes, rivers, tree coverage, small towns, landmarks, or anything that looks interesting.
Sometimes there is a specific place to visit or thing to see that dictates the route's general area.
If the route is more than 100 miles or so, you may need to force the route to go through a small town so you can get gas. For me, though, I really enjoy the small towns, so I like to have my routes go through them anyway.
When I first started building routes, I relied heavily on paper maps---my Delorme map books, etc. Now days, I almost never use paper maps anymore. In fact, I almost exclusively use the free, browser-based Route Planner at
http://www.marengo-ltd.com/map/. This tool is based on Google Maps. You may ask yourself, "how could a free, browser-based route tool be better than the commercial, professional MapSource software from Garmin?" EXCELLENT QUESTION! I'd like to ask Garmin! I think this is more an indication of how lousy the MapSource interface is rather than how great the Marengo tool is. I find the MapSource interface to be slow and cumbersome. Marengo on the other hand is fast and intuitive. Marengo has the HUGE advantage that with a single click, you can also view the satellite images of the area. I find this extremely helpful when I want to know "does that road really go through?" (In all map products, sometimes road map lines connect where the actual roads do not! Satellite images can clear that up.)
One thing (of many) safiri is better at than me is reading topo maps--you know, the maps with all the wavy lines indicating elevation. He is able to use this information to find the peaks and valleys and other interesting terrain features. (He and I both recommend Garmin's US TOPO maps product.) I need to get better at this, but what I do right now is use the satellite images (available right in the Marengo tool). I look for rivers and heavy tree coverage to guess where the valleys and interesting spots are.
The only time I use another map tool is when building routes in Kansas. The fact that the KS County PDF maps show the actual road surface type makes them fantastic. They even have an indicator that makes it easy to spot the "minimum maintenance" roads--which are a favorite of mine for dualsport adventure. These PDF maps also print relatively well on 8.5x11 paper for folding into your map sleeve or tank bag--in case you want additional resources during the adventure or if you do not have a GPS unit.
So I build my route in the Margengo tool. When complete, the tool opens a window with the route in GPX text format. I copy and paste this into a new file on my computer. Then I can open this file in MapSource---yes, with a Garmin GPS, you HAVE to use MapSource to actually interface your GPS to load the route. It's also the only tool to load the maps into your unit...and it does a very fine job of this.
Speaking of getting the actual maps into your GPS unit....some of you may not know but with MapSource, once you have a route--whether imported from a GPX, a GDB, or built right in MapSource, you can use the "select maps around route" feature to grab only the maps you need. It's also easy to just use the map selector tool and highlight a large rectangle around your route. Either way, you want to make sure your GPS Unit has the detailed maps loaded for the area you'll be riding.
Reasons to use one map tool over another usually revolve around price, detail, accuracy, and ease of use. I will say that Garmin's US TOPO maps are better than Google Maps when it comes to trails. However, based on my Ozarks trail experience, Garmin's trail data is severely lacking and the ones that do show are many times wrong. I'm really not faulting them for this--it seems impossible to keep up with all those trails. (Mike, how about the Rocky Mountain trails? Garmin's maps have a lot of those accurately marked?)
Something that I do for fun---not actually part of my route building--is load the GPX file in Google Earth.
http://www.google.com/earth/ (You need the Plus or Pro account to do this...Plus is $20/year.) It can be fun to view the route in Google Earth and "fly" through the route.
I hope this discussion and the experiences shared are helpful to some of you.