I find that negative thought happens when you don't think you can accomplish something. What I do is force myself to believe that I can accomplish the task in question. It's awesome because it makes me feel ahead of the game, like someone gave me the script for life ahead of time and I'm just following it. And if you think about it, that's how your brain works. It does what you expect it to do. If you expect failure, you'll get failure. If you expect success, you're brain will fill in the "how" by itself and sooner or later, you'll actually be succeeding.
That's the power of positive thought. And it actually works. It's not some hippy feel good shit, nor is it lying to yourself. Your brain will literally do what you expect it to do. It can't tell the difference between negative and positive thought or physical reality for that matter.