M
ark Forster points out that relying on willpower to get things done is not a good idea. Our will is a finite resource, and we shepherd its use carefully. We won't let ourselves use it for just any old thing, and needing to use it continually, over time, is a recipe for failure, as eventually we will give out; and once the will is broken once, it's must easier to break in the future.
Forster's overall point is that if we're not acting the way we would like, the problem is the system we're using, not our willpower. Human beings will, by and large, do the easiest thing. So, we ought to use our minds to develop a personal system for our lives that will make it easier to do the right thing, and harder to the wrong thing.
In Do It Tomorrow, this point is made in the context of productivity, but it applies more broadly to the moral life in general. If you want to stay loyal to your wife, for example, avoid strip clubs and flirting with coworkers; this will make staying true much, much easier. In the Catholic tradition, this is known as "avoiding the near occasion of sin". It is foolish to think that in order to be a good person, we should be able to put ourselves in situations of great temptation, and prevail due to our personal purity. We should never seek out temptation, and we should strive to make being good as easy as we can. We don't always have a choice, and we should choose well when we can.