If thoughts really do shape your character, then its essential we all maintain positive ones. I think realistically, it's not always possible to avoid negative thoughts. It's kind of like the pink elephant scenario. If I told you not to think of a pink elephant, I'm pretty sure your mind would still gravitate to a pink elephant. Thoughts tend to float around in that subconscious pool of space that we're not always consciously able to control. A good approach I've found is to 'curate information and filter what you expose yourself to.
Just like with a healthy diet, not all foods are good for you and some will do you harm regardless of how strong you are. Similarly, not all situations and information you expose yourself to are necessarily going to bring about positive thinking. We need to be mindful of what we expose ourselves to. To avoid negative environments, negative people and expose ourselves to positive friends, ideas and situations. We all have the ability to curate information like the songs on an iPod that change our mood. I find this can help facilitate the positive mindset that we're all looking for.