The mind has been conditioned to think through our daily needs and anything that has been conditioned for a long time, will run on its own momentum, as if having a life of its own. It takes a little effort and continuity to end that process. But understanding the way the mind works, its nature is such that something has to continue in it – there is no such a thing as a mind being blank.
As such a replacement will help to eliminate, if not reduce, unnecessary thinking. One can either replace it with awareness, by not forgetting to be in the present, or taking up the habit of observing what the mind is doing. Both techniques are useful as it grounds our attention back into the moment. You will observe that most of the unnecessary thinking has little to do with what is in the present. It is constantly either in the past or future.
Starting a fresh habit can be quite a pain, considering that what is being addicted to for so long has difficulty to change. Thus instead of trying to change the habit, try having the attitude of learning to watch the mind. Inject curiosity. Give it an interest to be aware or observe instead of trying to fix up the mind.
In this way, your approach becomes towards the direction of right attitude instead of wrong. By itself, incessant thinking is already a wrong attitude and using wrong attitude only makes the situation worse. One of the natural ways to inject curiosity into the mind is through questioning. The mind on its own nature directs itself through the question one poses. Of course, intelligent directed question is necessary if you wish the mind to follow. The mind is constantly in the know, even while it doesn’t know, as it knows it doesn’t know! Using this simple understanding, you direct the mind to where you wish it to go. Hence the purpose of the question is to bring about curiosity to know where or what the mind is doing. Its purpose is by no means to get an answer. It is more of an inquiry rather than questioning.
Imagine giving a question to the mind, say, where is the floor? Even without considering for a moment or even having the question completed, the mind already knows where it is. Are you aware of that? And if you pose a question that you are totally familiar already, which is an impossible task save being asked by someone, your mind will immediately come with a “don’t know” answer. It is only your lingering thought trying to figure out what the question is all about that probably disallows you to know that the mind already has an answer. That’s the nature of how the mind works.
Hence working on this nature of the mind is using it to check the mind. You may ask questions like, what is the mind doing? This will direct the mind to know itself. In the beginning you may find it difficult to know what the mind is doing. The trick is that you need not know what the mind is doing, the mind already knows. Again, it is your thinking that disallows being answered, which is already there, to be known. Hence each question posed is not for you to know or to get an answer but for the mind to go to where you had asked it. So probably you may have awareness coming back with, the mind is listening, or hearing, or thinking, or even imagining, or day dreaming.
It takes a little effort but the effort will be fruitful to reduce the unnecessary thinking mind. Long enough, you will develop a new habit of constant presence of what the mind is up to!