Here is something beautiful I read on getting rid of negative thoughts. Of course, the first step is becoming aware of them. You have to be on the guard, be conscious. There is no change without consciousness, without awareness. Now, for the technique.
Just tie a rubberband on your wrist. Whenever you catch yourself thinking thoughts that belittle your potential, thoughts that disempower you or thoughts that are negative, just snap the rubberband. You will hurt yourself, but the pattern will be broken and pain will be associated to the negative thought. Over a period of time, this will lead to change. So, go over and buy that pack of rubberbands today. Lets snap and break a few of them - and become more positive in the process.