In my opinion, the main keys to maintaining good health are to eat a healthy diet and to get a good amount of exercise.
Of course, there are other things that are important. A person should certainly not smoke if they want to be in good health. A person should not drink too much alcohol.
But beyond those things, I think that the most important things on a day to day basis are diet and exercise. By doing these things, people can avoid becoming overweight and can reduce the risk of such things as heart disease, hypertension, and diabetes.
I agree with pohnpei397, though I do have something to add. I think it is also important to be pro-active about your health. By this, I mean getting any screenings and tests done that are recommended for your age group. If you do have a health condition, then you need to stay on top of it and investigate the pros and cons of your treatment. This would also cover smoking, alcohol, and so on.