I know a lot of people use FranklinCovey’s builder or Stephen Covey’s (7 habits) exercise(s) – the one where you imagine what you would like everybody to say/think about you when you are dead (I, personally, read something similar somewhere else before, but I don’t remember exactly where). Other sites have mission statements builders, too, but the one that was really good for me was here
That’s because I already know what my values or principles are, but I needed something to help me translate that into doable things. Even if the mission that came out was not exactly what I expected while doing the exercise, it was a very good starting point and just needed some tweaking.
I wrote a spiritual and a personal mission statement, and it really helps.