One of the principles that I follow in my day-to-day life is to be an adult. Being an adult in this context doesn't mean having a house, a car, and 2.5 children; it means that you should try and act like a sensible, rational individual, and assume that others will too.
I often see people confuse doing what they believe to be the 'right' thing with what I'd consider to be the 'adult' thing. A good example of this is sick leave—when acting as a line-manager, I consider sick leave to be entirely discretionary to the individual (the 'adult' thing). Of course, most of the time you'll just have a cold or a virus, but sometimes you might just really not want to have to come to work. Maybe you've had some tough personal news, you're not sleeping well, or it's just a really fucking nice day. If you want to take a sick day, I'm not going to ask why.
Obviously this attitude is open to abuse. What if someone decides they don't feel like coming to work every week? What if they do it in the middle of an important project? I counter that these are not adult things to do. I trust that the people I work with are mature enough to not do these things, and act accordingly. Of course, in larger organisations practicality may intervene—it's not always the case that people will act in an adult way, and you begin to see byzantine sick-leave policies designed to protect the company rather than the individual (the 'right' thing). These are like long-winded coding standards and practices—after a while, you're just training your staff to be automata rather than thinking individuals, which diminishes the value of employing smart people.
Being a line-manager is another area that acting like an adult can help your outlook. Many people seem to confuse being a line-manager with acting in the company's interests. This is false—HR is there to protect the company from employees. Your job is the inverse—you're responsible for promoting your colleagues' interests, potentially over the company's. Does that new project look like a good training opportunity? Get your foot in the door and secure a place for someone on the team. Are they working a lot of overtime recently? Find out why, sort it out, then tell them to go home and get some rest. Take care of others by using your authority to help out—don't use it as a stick to beat them with.
If you're familiar with Dale Carnegie's book, 'How to Make Friends and Influence People', this outlook may not seem completely alien. One of the pieces of advice he offers is to assume that people make decisions to the best of their knowledge—by taking any other stance, you will only begin to undermine their decision-making and foster devisiveness. Taking an adult stance and assuming that others also act like adults precludes this kind of thinking.
Acting in an adult manner isn't just a professional concern; take working on a task with someone else as an example. It's not uncommon when doing something with a child or someone less experienced than you to see them try and do it in the 'wrong' way. Instead of always correcting them, or taking over the task, let them try their way (and fail if necessary). In the worst case they'll learn something from the experience, and in the best case you'll learn a new way to complete the task. (The obvious caveat here is to let them fail safely—if you're a skydiving instructor or an electrician this may not be the best teaching method.)
I've noticed that using this as a framing device makes it easier to figure out why you make certain decisions like those described above (and to figure out why some of them may be wrong). You can justify your stance more easily, and in many cases it makes it easier to say no to options that don't fit. As with any technique, once it becomes ingrained you don't really have to think about it to put it into action.
So the next time you have to make a decision that you're not sure of, ask yourself: What is the adult thing to do?