When I have a specific route in mind between 2 destinations, I find it easiest to simply pick a few waypoints along my preferred route and plot the route through those waypoints. I haven't had any issues with my older Nuvi 1490 failing to follow my preferences except in the rare occasion when there is no other reasonable option.
The preferences chosen are not an absolute. They are preferences, and if the receiver sees that a route following them goes excessively far out of the way, it will tend to select what it sees as a better path. The user has to stay in the game and pay attention. A GPS is a great tool, but it's not a substitute for human intelligence.