Some advanced routing codes: (these will work with any airline code)
UA,UA+ 2 or more flights on United
AA Single flight on American
AS,UA First on Alaska, second on United
|ITA Matrix Airfare Month Long Search Example|
I usually use Expedia to buy flights found on ITA Matrix. The trick is to just be specific with departure time and airline preference in the advanced search function. If that fails, I use Hipmunk to book a fare rather Expedia or Travelocity. Hipmunk doesn't 100% of the time either. Sometimes the airlines just won't sell it to you.
When doing a mileage run (trip just for miles) search like the one pictured, be sure to uncheck the Allow Airport Changes field. Also set the Length of Stay field to 0 so you don't need a hotel; a 0 length of stay will include overnight flights in the results.