Collosians 3:2 Set your minds on things above, not on earthly things.
Eventually your spiritual progress will lead you to the point where you actively control your thoughts. Actually, this is essential to spiritual progress and development of your will power, but it becomes obvious how important it is after a certain degree of development, and much inner work.
Jesus has told us that it is no sin to think whatever you want. (GGJ 7:36). Sin may arise, however, if you take pleasure in disorderly thought - then your will and desire joins with it, and in that lies the danger suggested in Proverbs 4:23 "Above all else, guard your heart, for everything you do flows from it." Action is preceded by thought and then will and desire. As Jesus says in GGJ 7:36 the thought is the seed of the fruit, so as the seed will be the fruit. So it is essential that we learn to regulate our thoughts, wishes and desires - in fact in doing this we are shaping the very inner core of ourselves.
I suggest avoiding disorderly thoughts altogether - they are suggested to us so frequently, and with such persuasion - for those with a worldly outlook - that in many of us they have become a habit which is hard to break, and also the taking of pleasure in them. Thus I suggest an important and useful exercise is to take control of your thoughts and be able to direct them to good and orderly things, when ever you wish - and particularly when confronted with encouragement or enticement to disorderly thoughts. Taking control of your thoughts allows you then to also develop taking control of your will and desires - and as such you become increasingly a master of yourself. Thoughts are powerful. This sentiment seems to be reflected in the following extract from "God Calling" edited by A. J Russell:
At One With God
To dwell in thought on the material, when once you live in Me - is to call it into being.
One with Me. I and My Father are one. One with the Lord of the whole Universe!
Could human aspiration reach higher? Could man's demands transcend this? One with Me.
If you realize your high privilege, you have only to think and immediately the object of your thought is called into being. Indeed, well may I have said, "Set your affection on things above, not on things on the earth."
To dwell in thought on the material, when once you live in Me - is to call it into being. So you must be careful only to think of and desire that which will help, not hinder, your spiritual growth. The same law operates too on the spiritual plane.
Think Love, and Love surrounds you, and all about whom you think. Think thoughts of ill-will and ill surrounds you, and those about whom you think. Think health - health comes. The physical reflects the mental and spiritual.
"For as he thinketh in his heart, so is he." - Proverbs 23:7
"We are what we think. All that we are arises with our thoughts. With our thoughts, we make the world." - Buddha