Passively observe your thoughts for about half an hour. Sit quietly and watch where they go without judging them. Observe your preoccupations, ideas, worries and hopes. Do this once or twice a day for about a week.
Make a list of things you like about yourself and things you'd like to change about yourself. Add to it each day for several days. Include thinking patterns as well as more concrete goals. For example, you might want to stop worrying that people don't like you as a negative thinking pattern and also meet more people as a concrete goal.
Establish habits that will help you reach your goal. If you want to overcome a negative self-image, for example, make yourself go out a few times a week and socialize with people who have a good view of you. It may take some time for your mental focus to catch up with your new habits, but your mind will come around.
Observe your thoughts passively every day. As you get better at observing your thoughts without judging them, begin to still your mind. Picture your brain as a blank, blue sky, a peaceful stream or some other calming image. Practice daily until you can eliminate undesirable thoughts at will.
Visualize whenever your old mental focus starts to pop up. Stop what you are doing and clear your mind for a moment. Gradually, you will dwell less and less on your old, negative thoughts.