First of all, they are able to minimize their waste. Look at your house and you will see lots of waste, but basically they come from three different sources: 1) rests of what you have used, 2) environment (dust, for instance), 3) things you have acquired but which do not contribute to your household. Out of the three, the third one is the most dangerous. One lady in Brazil had about 700 tons of things, including rubbish, in her yard... Now, just imagine how much do you have within? How many situations, people, problems and other things that happened many years ago are still in your memory? They are still defining how you should live. They are still in your thoughts...
A person with the art of thinking will be able to discard anything that is not valuable for the self or for others. He or she will be able to have thoughts that are full of value.
The second thing is in relation to the quality of those thoughts. OK, perhaps you don't accumulate useless things, but what you have, is that of good quality? You will consider quality as a set of criteria such as life duration or resistance. In relation to thoughts, their quality depends on the level of the awareness you had it.
Many thoughts just pop in our minds, like pop-corn, with no visible cause. Their quality is normally related to the awareness of the moment; for instance, if you are really relaxed, it is possible that thought brings you the solution for your financial problems. But if you are about to be operated and you are stressed due to that, it is quite possible your thoughts will be dark, with lots of images of people dying in hospitals.
Other thoughts have a very clear cause and you can manage your awareness in that case. If you are very relaxed, but you need to be concerned with what your daughter is telling you it is possible that you will talk yourself into paying more attention to what she is saying. On the other hand, if you are about to be operated and you read about people who have died on the table, you can talk to yourself in a positive and encouraging way.
At both situations, quality of thought is more elevated if it is a positive, encouraging, caring one. The one who masters the art of thinking will be careful to cultivate her or his awareness so that thoughts with that quality emerge.
To master that art, the best think is learning meditation, as it helps you to monitor your thoughts and create those, which give you a sense of well-being. In a powerful meditation state, you can easily create a masterpiece in your mind.