Choose to be happy. Yes, it is a matter of choice. When negative thoughts enter your mind, just refuse to look at them, doing your best to substitute them with happy thoughts
Look at the bright side of life. It's a matter of choice and repeated attempts.
Choose to be optimistic.
Find reasons to smile more often. You can find such reasons, if you search for them.
Have faith in yourself, and believe that the Universe can help you.
Associate yourself with happy people.
Read inspiring stories.
Read inspiring quotes.
Repeat affirmations that inspire and motivate you.
Visualize only what you want to happen, not what you don't want.
Learn Learn to master your thoughts