The Freakonomics blog recently posted a study done showing that people will give up something to prevent the possibility of regret. In the experiment participants were asked to pick a lottery ticket at random and then were offered a bonus to exchange the ticket for a different one. Because the first choice was random, there's no reason not to accept the free gift, but most do not. This is an irrationality I see often. I think it comes from a misunderstanding of sunk costs. There are certain things that are out of our control, for example the lottery, so they shouldn't effect our decision making.
I use this idea regularly on my long commute to work. It's well known that commuting, along with being a terrible use of time, is very mentally taxing. One of the more frustrating parts of driving to work in the morning with a late bell looming over your shoulder, is getting behind a slow moving vehicle. That is, until I realized that most of my drive is a sunk cost. There is very little I can do on a road with one lane and very few places to pass. So when I drive, I don't try to go as fast as possible without getting a ticket, I try to limit the space between me and the car in front of me. As long as I'm right behind them, there's nothing else I can do. No regrets, no frustrations.