I’m a Steven Pinker fan, so was happy to read his recent review of the book Willpower for the NY Times.
A few highlights (emphasis mine):
What is this mysterious thing called self-control? When we fight an urge, it feels like a strenuous effort, as if there were a homunculus in the head that physically impinged on a persistent antagonist. …
In experiments first reported in 1998, Baumeister and his collaborators discovered that the will, like a muscle, can be fatigued. … Baumeister tagged the effect “ego depletion,” using Freud’s sense of “ego” as the mental entity that controls the passions. …
The “will” in willpower is not some mysterious “free will,” a ghost in the machine that can do as it pleases, but a part of the machine itself. Willpower consists of circuitry in the brain that runs on glucose, has a limited capacity and operates by rules that scientists can reverse-engineer — and, crucially, that can find work-arounds for its own shortcomings. …
Watch for symptoms of ego fatigue, because in that recovery period you are especially likely to blow your stack, your budget and your diet. For that matter, don’t diet in the first place, since it starves the very system that implements self-control. …
[Willpower] disasters [like Oprah's yo-yoing weight] reveal a limitation of the muscle metaphor: certain evolutionarily prepared drives seem to withstand even the most bulked-up powers of will. The authors note that people with the highest levels of self-control are only slightly better than average at controlling their weight, and they describe disturbing experiments that confirm the old saying “When the penis stands up, the brains get buried” (it sounds better in Yiddish).
Nice timing given this recent PaleoHacks thread on using willpower to diet.
It’s very interesting that the book’s authors think that this part of the brain can be strengthened “with small but regular exercises.” I suspect that eating a nutrient-dense diet helps too!
Weight Maven is written by Beth Mazur. Beth believes that obesity is more symptom than cause and that the real problem is our Western diet -- especially sugar, refined grains, and industrial oils. Beth writes about nutrition, ancestral health, & food policy. And cats! 
I am convinced that will power per se isn’t effective because the effort required to sustain the action soon seems out of proportion to the results netted. A month of concentrated energy doesn’t really solve the problem and takes a lot of mental effort and focus. That is actually why I’ve been so successful with primal eating…in a paradox of choice way of looking at it, I eliminated a category of choices (grains and dairy), but don’t put a lot of thought into the other categories and am free to choose from any of them. No will power required, once I made the commitment to get rid of those categories (a decision reinforced by my after-the-fact discovery that I do have a problem with grains and just never knew it!)…
Just caught up with this post-something new for the book stack! I feel like the willpower discussion is a fascinating one for a lot of related topics like depression and creativity (or lack thereof). Why is it so easy to form intentions and gain a reasonably-workable model of how to do something, yet so difficult to fulfill intentions, and put what we “know” into practice? Acknowledging that will power is itself depletable allows us to treat our self-improvement regimens and intentions with a little more compassion and realism.