I'll admit that I'm not one for New Year's resolutions, as I think one can change something right now if they really wanted. However I think it's nice to have that "new year, new start" kind of mentality. I used to make changes in workouts, nutrition, lifestyle, etc. at the beginning of the month, say in the middle of the year. I'd have a goal such as "cut back on junk food" or whatever, and I'd spend the rest of the current month thinking about how I would change my lifestyle to accomplish the goal, and once the new month rolled around I would spend the entire month doing the goal, such as "zero sweets."
Once the month passed I would include sweets again, but the four weeks or so of strict discipline would help me break the habit of overindulging. I would never go forever without sweets, or even a year, but a month was enough time to set a realistic goal and have a sense of accomplishment, while also allowing proper time for me to break the habit. After the month I would include sweets in moderate, though it would be much better than having constant cravings or overindulging like I used to.
This is the same idea the Velocity Diet uses to change nutritional habits. I've used this same monthly method of lifestyle change for things such as backing off regular workouts and including a ton of rehab/functional work to my training in order to kick off better workout balance. I've used it to get used to getting up earlier, or say get everywhere ten minutes early than required so after the month (when I wouldn't have to arrive THAT early) I would be used to planning my time more efficiently. It's good for pretty much anything.