As soon as you notice that negative thought has arisen, ask yourself "Where did that come from? Am I angry with him?" If you realize that you are, ask yourself why. And then give yourself a little Dharma lecture. You know being angry doesn't make you happy. You know it plants the seeds for more anger in the future. You know current enmities stem from past enmities and that if unresolved will only be worse in the future. And you know it wastes your energy. And makes you cranky. So lecture yourself. I often do so.
If it's not even someone you know or if you have no reason to be angry, you might ask ""Am I jealous?" If that strikes a chord, tell yourself to get over it. Obviously the other person has the right conditions for what they have accomplished. Figure out what the causes for those conditions are and if they're good, do them yourself. You can either spend your time envying others for what they have accomplished or spend it in planting your causes for future good conditions.
Or maybe that negative thought arose from fear. Fear of you or someone you care about losing something. This is yet another opportunity for a Dharma talk. Remind yourself that whatever happens to you and whatever you have has been determined by your previous thoughts, speech, and behavior. No one can take from you what you destined yourself to experience or have. Just as you cannot obtain something that you have not destined yourself to experience or have.
As soon as you detect a negative thought, figure out where it came from. Then determine the Buddhist principle that applies and give yourself a mini Dharma talk. In this way, you can eliminate your negative thoughts and increase the positive ones.