I think that my guiding principle is hope. No matter what happens in my life, hope is what enables me to transcend my circumstances, allows me to try to be a better person, and provides me with the motivation to improve the world.
I think the most important principle is what is often known as the Golden Rule, or Karma. Basically, treat others as you want to be treated. It requires empathy. Part of what makes you a good person is your ability to understand how another person feels.
I agree with the third post that honesty is the most important aspect in one's life. I try and tell the truth at all times and never mislead others (an important requirement of a teacher), and I wish that all people followed suit--especially politicians and other leaders.
I tend to favor the Golden Rule of "treat others as you would like to be treated." The Golden Rule really seems to incorporate almost every other ideal principle. For example, I would not like to be lied to so the Golden Rule should keep me honest. This is probably something we all learned as children. Although, many adults fail to practice such a simple ideal. Placing yourself in another person's shoes can have a great impact upon a person's behavior.
Act guided by the principle of what is in everyone's best interest. Dig hard to find the win-win; so much of the world is simply satisfied with the "win-lose." Ideal situations between parties are rarely found, but the attempt to come to consensus by honest parties yields not the ideal, but at least the best results.
Well, there are a number of contenders for the answer to this question. First and foremost for me, however, before anything else, would be self-respect. This is because if you don't have a level of self-respect, I can't see how you can have an easy life. This, if you like, comes before and is a foundation to other values such as honesty.
I like both of the previous answers, but I will offer a third: try to make a positive difference in the world before you die. Try to do anything you are capable of doing, however small, that will leave the world a better place than it was when you entered it.
I would say that honesty is one of the most important principles to live by, and I mean honesty with others and honesty with yourself. If you are honest with others you will live with the peace of a clear conscience and the knowledge that you doing the right thing most of the time. If you are honest with yourself it helps in your self-reflection and your ability to continuously improve yourself.
The most important principle is that you should live your life more for others than for yourself. That is, you should not be selfish and put yourself first. If you put yourself first, you end up being a stunted person. If you put others (family and friends in particular) first, you end up being happier and more satisfied.