Positive thinking helps your stress level and can improve your health. People who engage in positive thinking tend to experience lower rates of depression and better psychological & physical well-being. Positive thinking may even increase your immune system’s ability to resist the common cold. Here are some strategies for thinking in a more positive way:

• Check yourself. Take time throughout the day to evaluate what you’re thinking. If you notice that your thoughts are mainly negative, find a way to put a positive spin on them or think of something you are grateful for.
• Be open to humor. Allow yourself to smile or laugh, especially during difficult times. Finding humor in tough situations can reduce stress.
• Surround yourself with positive people. Choose to spend time with friends or family who are positive and supportive. Negative people may increase your stress level and lead to self-doubt.
• Practice positive self-talk. Treat yourself as you would treat a friend or family member that you care about. Be gentle and encouraging with yourself. When you find yourself thinking negative thoughts about yourself, respond with positive and encouraging affirmations.

http://www.mayoclinic.org/healthy-lifestyle/stress-management/in-depth/positive-thinking/art-20043950?pg=2