A good exercise is to analyze the kind of thoughts you have been thinking. Some will be spiritual thoughts that make a positive contribution to life, others will be thoughts about things that are not necessarily good or bad, and there will be thoughts that are definitely bad--and you realize this without anyone's telling you so. The quickest way to deteriorate or to degenerate is to allow your mind to be occupied with unworthy thoughts. We soon become what we think. Thinking good thoughts contributes to building character; thinking bad thoughts leads downward.
Jesus explained that the mouth really reveals what is in the heart: "The good man out of the good treasure of his heart brings forth what is good; and the evil man out of the evil treasure brings forth what is evil; for his mouth speaks from that which fills his heart" (Luke 6:45, NASB). What the conscious mind thinks on gradually sinks into the subconscious mind and becomes the building blocks, or material, for one's character. "For as he thinks within himself, so he is" (Prov. 23:7, NASB).
We can make a positive contribution to our subconscious mind by controlling the thoughts of our conscious mind. But when we think selfishly, covetously, jealously and lustfully, these characteristics will become evident in our character.
"And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect will of God" (Rom. 12:2).