Listen up airlines, I have a couple ideas for you.
1 - The first airline to do this will make a major media splash as well as good will (unfortunately Southwest won't be able to do it, though). Once the plane is loaded, have a random draw from the coach class to fill the remaining first class seats. It would be like winning a mini lottery. Seats are empty, why not give the coach people a break from the sardine environment in the back by moving a couple forward. Think of the word of mouth advertising and all of the media outlets will pick this one up as well - Free good advertising! All of those folks that play the lottery or gamble will want to fly your airline just for the thrill of the possibility of getting a better seat.
2 - Sell coupon space on the back of the seats. Attach a clear pocket on the back of the seats and sell the space. You've got a person's eyes for at least an hour, that is an amazing amount of time in the ad world. Plus the passenger gets to walk away with something tangible in their hand.
3 - On those planes with the individual video/gaming units on the backs of the seats, why not add coupon capability. The routes that these planes fly are usually several hours to overnight flights. Why not set up a way so the passenger can browse coupons by destination? Let the passenger enter an email address and then email the selected coupons to them to print out. If you wish to make it even more valuable, put a simple kiosk at the gate or baggage claim so that they can print them out once they arrive. All of this can be done as a revenue generating situation. And once again, think of the eye time that the advertisers get!
Maybe if the airlines could offset some of their costs through selling ad space, they could get away from this nickel and dime mentality that is so popular now. I recently flew on a carrier (who will remain nameless) that did so much selling that I felt like I was in one of those timeshare meetings.