Practice, practice, practice, mindfulness. For more information on the concept of "Mindfulness" please visit our "Manage Stress" page.
Being mindful helps you stay in the moment. When you find yourself thinking bad things or have unwanted thoughts, stop, breathe, and be in the moment.
Notice your surroundings, and notice how you are behaving. Identify what triggered these thoughts. After this, create a strategy/plan to help yourself stop these thoughts. Do you need to stop doing something in particular? Is it a specific person bothering you, or an ongoing event? Find ways to stay away from these triggers.
Having power in controlling your thoughts allows for you to basically control everything. Your thoughts are also carried with "Limiting beliefs". Some people may think that certain things are impossible, but eliminating those thoughts helps combat those "Limiting beliefs", ultimately allowing for you to conquer what you want to achieve!