In general, will is a very useful thing to have. After all, it allows a person to overcome factors that would make his decisions for him, such as pain, fear, anger, fatigue, lust or weakness. I would, of course, be remiss to not mention that the will can be used to overcome generally positive factors such as compassion, love and mercy as well. The will, as Kant noted, can apparently select good or evil with equal resolve. However, I will set aside the concern regarding the bad will and focus on training the will.
Based on my own experience, the will is rather like stamina—while people vary in what they get by nature, it can be improved by proper training. This, of course, nicely matches Aristotle’s view of the virtues.
While there are no doubt many self-help books discussing how to train the will with various elaborate and strange methods, the process is actually very straightforward and is like training any attribute. To be specific, it is mainly a matter of exercising the capacity but not doing so to excess (and thus burning out) or deficiency (and thus getting no gain). To borrow from Aristotle, one way of developing the will in regards to temperance is to practice refraining from pleasures to the proper degree (the mean) and this will help train the will. As another example, one can build will via athletic activities by continuing when pain and fatigue are pushing one to stop. Naturally, one should not do this to excess (because of the possibility of injury) nor be deficient in it (because there will be no gain).
As far as simple and easy ways to train the will, meditation and repetitive mental exercises (such as repeating prayers or simply repeated counting) seem to help in developing this attribute.
One advantage of the indirect training of the will, such as with running, is that it also tends to develop other resources that can be used in place of the will. To use a concrete example, when a person tries to get into shape to run, sticking with the running will initially take a lot of will because the pain and fatigue will begin quickly. However, as the person gets into shape it will take longer for them to start to hurt and feel fatigued. As such, the person will not need to use as much will when running (and if the person becomes a crazy runner like me, then she will need to use a lot of will to take a rest day from running). To borrow a bit from Aristotle, once a person becomes properly habituated to an activity, then the will cost of that activity becomes much less—thus making it easier to engage in that activity. For example, a person who initially has to struggle to eat healthy food rather than junk food will find that resisting not only builds their will but also makes it easier to resist the temptations of junk.
Another interesting point of consideration is what could be called will surrogates. A will surrogate functions much like the will by allowing a person to resist factors that would otherwise “take control” of the person. However, what makes the will surrogate a surrogate is that it is something that is not actually the will—it merely serves a similar function. Having these would seem to “build the will” by providing a surrogate that can be called upon when the person’s own will is failing—sort of a mental tag team situation.
For example, a religious person could use his belief in God as a will surrogate to resist temptations forbidden by his faith, such as adultery. That is, he is able to do what he wills rather than what his lust is pushing him to do. As another example, a person might use pride or honor as will surrogates—she, for example, might push through the pain and fatigue of a 10K race because of her pride. Other emotions (such as love) and factors could also serve as will surrogates by enabling a person to do what he wills rather than what he is being pushed to do.
One obvious point of concern regarding will surrogates is that they could be seen not as allowing the person to do as he would will when he lacks his own will resources but as merely being other factors that “make the decision” for the person. For example, if a person resists having an affair with a coworker because of his religious beliefs, then it could be contended that he has not chosen to not have the affair. Rather, his religious belief (and perhaps fear of God) was stronger than his lust. If so, those who gain what appears to be willpower from such sources are not really gaining will. Rather they merely have other factors that make them do or not do things in a way that resembles the actions of the will.