I was reading the April Self Magazine, and saw a great tip for how to get rid of those negative thoughts that plague us sometimes. They said to "erase a negative thought by shaking your head no."
Research shows that this can help rid your mind of the thought so you can move on to something more pleasant. Simple and easy. I like it.
I think viewing some of the positive quotes we have about friendship can help as well. Like anything in life, you have to work at keeping your thoughts in a good place, but it's worth it.