We mentioned earlier [Day 18] that willpower is like a muscle whose strength ebbs after a long workout; it is like a muscle in another way as well: While muscles become exhausted by exercise in the short term, they are strengthened by regular exercise in the long term. Perhaps regularly exerting "self-control" improves willpower "strength" as well.
In fact, the Chinuch to the mitzvah of "Velo sasuru" explains the well known concept of "mitzvah goreres mitzvah; aveirah goreres aveirah" - the performance of one mitzvah "pulls one" )predisposes one( to perform another mitzvah, and the performance of an aveirah predisposes one to perform another aveirah - as follows:
If you decide to follow your physical urges one time, you will automatically be drawn to follow them many times; and conversely, if you are disciplined and control your desires by shutting your eyes from seeing immorality, it will be easier for you to continue doing so in the future. For self-indulgence draws the flesh like wine attracts alcoholics ... the more they drink, the more they thirst for. If they would only drink even a [single] cup of water instead, their burning desire would diminish, it would make their life easier.
So it is with regard to [guarding one's eyes]: As long as one continuously satisfies his desires, he empowers his yetzer hara. But when he disciplines himself [the yetzer hara diminishes and] he will rejoice every day [as his nisayon decreases correspondingly].
Recent scientific research has reached a similar conclusion.
In one study, researchers found that smokers who practiced self-control in another area of their lives - avoiding sweets or regularly squeezing a handgrip - for two weeks were more successful at quitting smoking than control subjects who performed two weeks of regular tasks that required no or little self-control, such as writing in a diary.
In another study, volunteers were assigned to a two-month program of physical exercise - a routine that required willpower. At the end of two months, participants who had stuck with the program did better on a lab measure of self-control than did participants who were not assigned to the exercise regimen. That's not all. The subjects also reported smoking less and drinking less alcohol, eating healthier food, monitoring their spending more carefully, and improving their study habits.
Regularly exercising their willpower with physical exercise, it seemed, led to better willpower in nearly all areas of their lives.
Remember, the job gets increasingly easier. We just must train ourselves to do what is right.