One of the most recognized phrases within the LDS Church's mainstream culture is, "Hum your favorite hymn." This motto is part of a primary song that children learn, and the overall idea is described thus in that song:
If on occasion you have found your language is in question, Or ugly thoughts come to your mind, then here's a good suggestion: Just hum your favorite hymn. Sing out with vigor and vim, And you will find it clears your mind. Hum your favorite hymn.
Before you say an angry word, remember you'll regret it, For once it's said the harm is done, and people won't forget it. Just hum your favorite hymn. Sing out with vigor and vim, And you will find it clears your mind. Hum your favorite hymn.
First, it's important to emphasize that this is good advice to handle those times when "evil" or "improper" thoughts come to mind unbidden. It is good advice particularly for those, like children, who have not found a way to keep those thoughts at bay in the first place. It is, however, a process of engagement - not a process of replacement, so it is only a "preparatory" or "initial" activity.
By that distinction, I mean that relying on humming a hymn, while effective to a degree, assumes that there will be regular thoughts that come to mind that must be engaged - battled in some way. There is no recognition of any need to change the mind - or the environment in which the mind operates - in such a way that the mind itself generally keeps such thoughts from developing to the extent that an engagement mechanism is necessary. In other words, rather than merely engaging an alternative activity every time an improper thought crosses the stage of our minds in order to engage and reject the thought, a replacement approach would be to condition the mind to keep the doors to the stage locked and not allow the improper thoughts to walk onto the stage in the first place. After all, "idle hands are the devil's workshop" - and, likewise, idle minds are the devil's playground.
I'm not certain at the personal level that perfect (full, complete, fully developed) elimination / replacement is possible, since I have not reached that state yet; hence, the acknowledgment that engaging by humming a hymn is a good first step and training model when unbidden thoughts come to mind. However, just as a "Fresh View of Repentance" involves "a change of mind, i.e., a fresh view about God, about oneself, and about the world", dealing with evil thinking in totality must be attempted by changing one's very mind.
How is that possible? How can a mind be changed so fundamentally that it simply ceases to think anything that is evil - either in a purely moral sense or in the sense of not being harmful in nature, as has been discussed in my previous New Year's Resolution posts this month?
Fortunately, there is a simple answer to this question - one that is almost as omni-present as the humming of a hymn. In fact, the humming of a hymn actually is one possible manifestation of this answer - namely, filling one's mind with good thoughts and leaving no room for evil thoughts to crowd in and need to be engaged. This is why humming a hymn is a good practice on those occasions when an evil thought does intrude - but there is much more that can and should fill our minds than just the words and tunes of hymns. I didn't say it is an easy way - merely that it is simple.
Any good and virtuous thought - any contemplation of a positive nature - any act of service (both in the planning stages and in implementation) - any discussion of value - any study of good books - any listening to uplifting or calming or inspiring music - ad infinitum; all these things can fill our minds and bodies with proactive good and keep unworthy thoughts from intruding.
The best approach, therefore, is not to allow "down time" in such a way that unwanted thoughts encroach upon our minds. Rather, even in those necessary down times, we can fill our minds with goodness and right.
How that is accomplished on an individual level must be, of necessity, an individual decision - since exactly what will inspire and uplift and edify and teach and expand spiritually and intellectually and emotionally (what will keep evil thoughts at bay) will vary from person to person. That, in a nutshell, is the most important reason why humming a hymn cannot be the default mechanism on which everyone relies - and it is why full replacement will not occur without proactive, intentional planning and conscious choice.
We are not robots to be programmed; we are souls that need to grow toward perfection. Humming a hymn can take us only so far - and that distance will vary for each person.