Believe it or not, the words you speak are commands given to your mind. You may think it is crazy to talk to yourself, but this is the simplest way to get you to take action on what you want to do or want to become.
Whatever your vocalize, your mind will believe that it is true and will start storing information of your surroundings that are related to those beliefs. For example, have you ever noticed that when you are interested in a particular item, you start noticing that more and more poeple have that same item. It could be a car. Once you are interested in a particular car, you will notice that same model more often than before.
This happenes because your subconscious mind is looking for that interest. Your subconscious mind sees many times more through your eyes than you can consciously be aware of. What is registered and stored in your mind depends on what you belive and/or have told yourself to be true, and your interests.
Give yourself verbal orders and your mind will act on them. Compliment yourself and you will belive it. You can be more assertive, you can improve your self-esteem, you can do great in an interview, you can give the best presentation, you can enjoy anything, as long as you tell yourself over and over, day after day.