Subconscious Mind?

Right now I am reading a book called Think and Grow Rich by Napolean Hill. It's fairly old but has some great ideas. One thing I found that was very interesting had to do with the subconscious mind. The author states that if you persistently tell yourself something, eventually your subconscious will believe it to be true and will act on that thought. For example, if you tell yourself over and over that you are going to ace an upcoming test, eventually your subconscious will believe it and force the body to do everything possible in order to make that idea a reality. On the subversive, if you tell youself that you will fail over and over, you will not study and will not be prepared for the test.

I know the idea can seem far-fetched, but according to Napolean Hill, hundreds of studies have been done that prove this to be accurate. He recommends that you write down a sentence to say out loud twice a day ("I am going to ace my Spanish test") and to focus on that idea as much as possible throughout the day. So think of something you want to accomplish and convince yourself that you can do it.