Jonah Lehrer wrote an excellent article for the Wall Street Journal on why New Year’s resolutions fail.

Quick overview of reasons:

• The more overloaded our brain is with ‘stuff’ (including resolutions we need to keep track of), the weaker its willpower is. We need to spread our resolutions across the year.
• Just like a muscle, our will-power has limited capacity – don’t make it do too much or you’ll lose self-control.
• Again, just like a muscle, exercising will-power uses up energy (glucose), so beware if you’re trying to lose weight as a resolution – starving yourself will reduce your will-power.
• When you reach the inevitable point of temptation, distract yourself rather than trying to power through.
• Will-power can be trained and training it in one domain (eg posture) increases will-power in others

I really recommend the article, it’s great stuff. You can also find Jonah’s blog here.