This is a habit of mind I live by in my professional life. I think it is very important for people who work together to feel like there's value in getting "mutually beneficial results." I try to keep this in mind with working with all staff members, from classified staff to fellow teachers to administration. I think this leads to healthy relationships and positive attitudes, which helps all staff to focus on what's best for kids.
I think rather than "teaching" this habit to my sons, I will present them with scenarios and we'll talk about how to get to a win-win outcome in each situation. We'll address issues at home (for example, we could discuss raising their allowance but increasing the chores they do at home - how is this a win - win?), at school (for example, when working on a school project what can you do to make sure each member of the team participates equally), and at work (a friend wants you to work for him a couple of hours the next day - how can you make this a win - win situation?).
I think the process of make this "real" for them will help them to learn this habit.