Resist Evil Thoughts 
I received another important truth as a young man in a meeting in the tabernacle when President David O. McKay was giving one of his memorable addresses.
In his talk he made the statement, "Sow a thought, reap an act, sow an act, reap a habit, Sow a habit, reap a character, sow a character, reap an eternal destiny."
Then he proceeded to explain that no one ever commits any sin or wrongdoing unless they have thought about it ahead of time; and it is the mulling over or retaining in our minds the evil thoughts that ultimately result in our carrying them out.
This statement worried me because my friends would occasionally tell dirty jokes, and the ideas expressed would stick in my mind and I would mull them over.
I did not want to do the things that they suggested, remembering that they were contrary to the will of the Lord. Again, having an understanding father was a real asset to me because, as I explained my concerns to him, he told me that it needn't be a problem if I recognized that you cannot think of two different things at the same time.
He said that while we can't keep out of our sight and hearing things that are evil, we can kick them out of our minds if we are willing to think of something else to displace them, like being in some special place doing the kind of thing we would most rather be doing at that particular time. I decided to test this.
The next time such a thought was planted in my mind, I began to think of being carried in an airplane over Mount McKinley in Alaska with all my ski gear on and with a parachute attached.
I would bail out of the plane and gracefully land on top of that majestic mountain, take off the parachute, and then begin to ski down through the magnificent snow fields, spraying powder snow up behind me as I made elegant turns and enjoyed the special joy of making "gala-enda springs" off mighty cliffs, landing safely in the snow below as I made my way down the mountain.
Needless to say, the original thought vanished. As a matter of fact, as I've recounted this technique I have found it difficult to refocus on the basic theme of my talk!
Other ways of accomplishing the same thing, advocated by many of the Brethren, include thinking of a hymn that you love very much and running the words and the music through your mind, or reciting a scripture that you have learned or are trying to learn.
All of these techniques are valid in that they displace the original thought and enable you to avoid thinking so long about something evil that you will do it.