Willpower is an equilibrium between two distinct drives, one of which is a drive to act, and one of which is an inhibitory drive. The first step to growing willpower is to make sure your lack of willpower isn't caused by a biological problem or a mental health problem. Before struggling too long to grow willpower, have a physical, have your thyroid level checked, and get a mental health evaluation.
The reason you can build willpower is that it is an equilibrium between the parts of your brain that produce fear and anxiety and the parts of your brain that produce good feelings. Your right prefrontal cortex produces negative emotions, and your left prefrontal cortex produces positive emotions. This circuitry is linked up to the circuitry that primes your body to move. Assuming no mental or physical illness, willpower is simply a measure of the relation between these systems.
We know that monks who meditate for 10,000 hours have much greater gamma ray function. Gamma rays underlie large scale brain synchronization. These monks also had a different relationship between the left and right prefrontal cortexs (corti?). In monks, the positive emotion producing left prefrontal cortex was much more active than the right. Also, they were much more primed to move.
Meditation, particularly certain kinds from Zen Buddhism and Tibetan Buddhism, are scientifically proven to work on this, to such a degree that neuroscientists haven't seen some of those neurological states in normal people. From studying these monks, we know what to do.
It's scientifically proven that you can grow willpower by getting a physical, and a mental health eval, and then meditating. It's the best, most efficient way. Everything the nyt article lists as a way of increasing willpower is an example of an activity where you must be continually bringing your attention into the present. I don't want to sound like some stupid hippy, but meditation can be the most concentrated way to practice this focusing of attention, and directing your focus in certain Tibetan buddhist ways hits all the main parts of increasing motivation.
I consulted http://www.urbandharma.org/udharma8/monkstudy.html to refresh my memory on the monk neurology study.