Here's a tip that often helps. First, write down your reasons for wanting to stop smoking. And then turn them all into positive reasons. For instance, if one of your reasons for stopping is to "not get ill" then change that to "enjoying good health". Your list will probably include reasons such as to be fit and energetic, and to save money and enjoy more holidays.
It's important to be positive, but also to think about what you might find difficult and how you are going to overcome those difficulties. What will your contingency plan be if you have a bad day? How will you deal with it without resorting to thinking about smoking?
For further ideas and advice and ways to stop smoking, visit the substantial resources section of this site - Stop Smoking Resources