Here's another but risky option. I booked the cheapest fare from SFO to PHL which (surprise!) turned out to be a codeshare with US. First time to fly codeshare other than Lufthansa. I was appalled when I learned all the nuances of codeshare, inc no upgrades. Soooo, I went late (flight at 850 AM) to the airport and did standby to PHL totally on UA metal (much more expensive flights when I was booking) AND got an upgrade for husband and me (1K's). Told them we had a power outage which turned out to be true! On the return, we went early and not only got UA metal but a freebie upgrade. If you do that, you get all the UA bonuses, etc as if you booked it in the first place. I am not sure I would do it again. I suspect I will simply avoid codeshare. I also checked with UA the night before each flight to see what the load was on the flights I wanted to fly.