QuietLion is correct on just calling 24 hours in advance. Actually, since there is inevitably a 5-10 minute delay on Alaska's 800 number; make that 24 hours and 5 mins in advance of the flight departure time.

I just flew three roundtrips between Phx and Portland over the past three weeks. Including just now returning on an MD-80 flight. Heh, heh. Of my six calls to secure an upgrade, I was successful two times. Definitely worth the $40. AS is a good airline to fly, I am only sorry that they do not fly to many of the places that I do. Good Luck!