I usually have a final destination, and a rough idea of how I want to get there. Then I just try to find the smallest roads on the map that don't seem like they will be impossible to navigate. This usually works out, but I have definitely changed my route mid day before if I end up on a busier road than I want to.

You can also google the area you are going through and try and see if you can find someone else that has posted a route online.