If you fly Icelandair, they started a new program a few weeks ago called
CLASS UP; 10 days prior to your flight, you will be sent an e-mail which will offer the opportunity to bid for an upgrade.
Lets say you have a coach seat; you could get an e-mail letting you place a bid for an Economy Comfort or Saga class seat. All you would pay is what you bid, if accepted, for the better seat.
Not all flights have this offer, and they may not accept your bid, but I find it an interesting concept.
Icelandair is looking to sell those seats in EC and Saga at the last minute, if they like your offer.
There is additional information about CLASS UP in the FAQ on Icelandair.com. Look in the section labeled "On Board", then go down that section about 3/4 of the way, and you can see the questions answered about CLASS UP.
Perhaps a chance to get a much better seat for a much lower price.....