If we're going to spend that long every day doing something, we might as well do it as well as possible. Most people are prevented from improving the way they think not through actual inability, but by beliefs which cause them refrain from trying many things because they assume improvement is impossible. In order to make you more likely to believe what I am saying without actually having to present meaningful arguments for my claims, I shall present this in the form of debunking several myths which limit many people's ability to improve their thoughts.
Myth: You can't control thoughts that happen automatically.
Sometimes you have thoughts which aren't deliberate. Perhaps you get afraid when you're somewhere high off the ground, you get angry every time you encounter a certain person, or you experience an irresistable temptation to eat junk food whenever you see it. You don't particularly want to think these things, but they happen anyway, and you therefore conclude that they're out of your control.
Nothing could be further from the truth. These thoughts are just as much within your control as any others. The difference is that the method by which you control these thoughts is not by willing them to change.
You can't make your muscles stronger by wishing for them to be stronger, yet few people would say that strength is out of one's control - most people know that if you want to be stronger, you can affect the situation by doing something like exercising more. So it is for control of your automatic thoughts - you can't control them by simply wishing for them to change, but this doesn't mean you can't control them at all.
Myth: You're limited to one way of thinking about things.
We often tend to assume that we can have only one way of thinking about the world. We think that if we want to change it, we must do so by replacing it with another way which we consider "better."
It's actually possible, however, to have many different ways of thinking about the same thing which focus on different aspects of the situation. Of course, when you have more than one option, you have to take a moment to choose between them, but this is usually a small price to pay.
There's a legend about a detective. This detective was excellent at what he did - he ferreted out cunning criminals all day, and was given accolades and promotions by the bucketful. He was so good and suspecting people, however, that he had forgotten how to interact with them in other ways - and one day his wife divorced him because she was tired of his snooping and accusations.
Reality can be like this legend, but it doesn't have to be. When you learn a new way of perceiving the world, you don't have to use it all the time. You can choose to sometimes use the new way, and sometimes use the old way. There isn't even any magic technique required - all you have to do is remember that you have a choice, and make it.
Myth: Things which seem hard now will always be hard
Sometimes we find that something is frustratingly hard and takes too much effort. Perhaps, for example, you once tried to make a conscious effort to improve your posture or speech, but became aggravated by how much of your concentration it required and gave it up as something which was not worthwhile. On the other hand, perhaps you forced yourself to learn a skill such as touch-typing or speedreading, and eventually observed how it became automatic and easy, in spite of how frustrating it was at the beginning.
What you need to remember is that almost everything can become automatic after you practice it for awhile. There is a natural tendency to initially become discouraged by the amount of effort required to do something, but remembering that almost everything becomes automatic with practice should help you keep at it.