United has just announced that MileagePlus members can earn double miles on select West Coast flights now through the end of 2013.
From the United Mileage Plus Announcement:
For a limited time, you can earn double award miles when you fly nonstop between Seattle and any of the following cities: Los Angeles, San Francisco, Portland (Oregon) or Anchorage.
To qualify for this offer, register and purchase before December 31, 2013, and complete your travel by May 31, 2014.
To register for the promotion, all you need to do is head over to the United Airlines site and register. If the provided link does not auto-populate the promotion code, just enter it manually. The promotion code for the double miles on West Coast flights is: TB9M61.