A timeless simple message from one of the best teaches I have ever encountered, Boyd K Packer. Elder Packer tells a story that illustrates the importance of controlling ones own thoughts.
I'll add my own quick thought. Often our minds are not ready for the trials of life and we find our thoughts going in different directions because our mind is not properly prepared to deal with the situation at hand. I have noticed that when my mind is being prepared by studying and learning good things, my thoughts are much more easy to control. In a world filled with temptation, we must seek to prepare ourselves before temptation arrives.