Guidelines:
Begin at the center:
Use all these factors in writing your statement:
Security:

your sense of worth, your emotional anchorage, your self esteem, your basic personal strength or lack of it.
Guidance:

your source of direction in life and your internal frame of reference that interrupts what's happening out there, your standards and principles that govern your decision making.
Wisdom:

your perspective on life, your sense of balance, your understanding of how the various parts and principles apply and relate to each other
Power:

your capacity to act, your strength to accomplish something. Your vital energy to make decisions and it also includes your capacity to overcome the deeply embedded habits and to cultivate more effective ones.

When these four factors are present together, harmonized by each other, they create the great force of noble personality, a balanced character, a beautifully integrated individual.

Your paradigm is the source from which your attitude and behaviors flow. A paradigm as Peach mentioned in one of the comments is like a pair of glasses; it affects the way you see everything in your life. If you look at things through the paradigm of correct principles, what you see in life is dramatically different from what you see through any other centered paradigm*.

Identify roles and goals;

your role as an individual, as a student, as a father or a mother, a manager and so forth. Write a statement for each, and set an affective goal that focuses primarily on results rather than activities.
Writing your mission in terms of the important roles in your life gives you balance and harmony. It keeps each role clearly, before you. You can review your roles frequently to make sure that you don't get totally absorbed by one role to the exclusion of the others that are equally or even more important.
Use your whole brain:
Although people use both sides of the brain, one side or the other tends to dominate in each individual. The ideal would be to cultivate and develop the ability to have good crossover between both sides of the brain so that a person could sense that the situation called for and then use the appropriate tool to deal with it. We live in a primary left brain dominant world, where words and measurements are enthroned, and the more creative, intuitive, sensing, artistic aspect of our nature is often subordinated.
We manage from the left side, but lead from the right side. Therefore a little tabbing to the right side of our brain is necessary to come up with sound, satisfying mission statements.
And btw; the first image your brain perceived when you did the young-old women test I posted in one of my previous posts, shows which side of your brain is predominant.

An exercise to tab the right brain:
1-Expand your perspective by visualizing a scenario in the future; your graduation, your wedding, your funeral, etc
2- Expand your mind by visualizing in rich detail the entire scenario, involving as many emotions and feelings as possible and involving as many of the senses as you can.

And there is another exercise which the author did not mention in his book but science had proved its efficiency in cultivating a successful crossover between both sides of the brain; this exercise is called the Alternate Nostril Breathing technique which is essential in most yoga classes.
Click on this link if you are interested to learn more about the technique.
http://www.abc-of-yoga.com/pranayama/basic/viloma.asp


*Most of us are unintentionally driven through a self centered paradigm or paradigms that is exhausting our energies and time at the cost of other important things in our life; money centered paradigm, religion centered paradigm, family or spouse centered paradigm, pleasure centered paradigm and position or job centered paradigm. etc