Our trip to New Zealand in October of 2013 was on board Air New Zealand. We knew to upgrade to a preferred seat which gave us a little more leg room. It didn't cost a whole lot and you have to do it early in the game because they go fast.
The other thing Bob found was the opportunity to bid on an upgrade to Premium Seats which were like a business class. He loved the site where you placed your bid because it gave you odds on whether or not it was a good bid. He did some calculations on what it would cost outright to upgrade and halved it and bid just a little under that. We didn't get upgraded on our way out to Auckland but received notice as we were ready to leave that our bid was accepted going back home. A look around the cabin we were in told us Bob could have bid less. The seats were only half full.
Now I see that American Airlines is trying out this idea. You can now bid for an upgrade to first class on their flights. You go to the American Airlines site and enter your offer. You must give a credit card number but it will only be charged if your bid is accepted. Then they will notify you within 24 hours of departure if your bid is successful.
Who's next? Going once, going twice. . .