Is there a list of directs and preferred routes within USA airspace?
In the US, the National Flight Data Center (NFDC) publishes a database of preferred routes.  You can query it or download the entire database.

Here is the result of entering only BWI to PHX:



A practical resource for planning a real flight is to review the routes of previous flights.  I browse the origin and destinations, or nearby airports to my flight and select a similar aircraft to which I am flying. On FlightAware, the route is shown in the lower right.  Here's an example: