It's a story so common that it has become a cliche. A person decides that for the New Year they will take better care of their health, so they sign up for a gym, go on a diet or buy a treadmill. In the beginning they are motivated, but then they tail off, and a few months later are back to their old habits.The process is so predictable that a significant portion of the fitness industry is built on it, as you'll see from all the gym and diet advertisements in the next few weeks.
Thankfully, there is a better way, and it hinges on your mindset. To get healthy and stay that way, you are better off thinking of fitness as a way of life as opposed to a series of goals that once hit mean you are done. In other words, stop planning on "losing 15 pounds before the spring" and start thinking along the lines of "In the new year I will be good to myself by cutting out processed carbs that sap my energy."
They key is to find an activity you enjoy, whether its finding a trainer that keeps you motivated or learning fun healthy recipes - and then to give yourself a series of manageable goals. If you decide to lose 15 lbs in a month or go from no exercise to 4 times a week you are unlikely to succeed, and even if you do, results that come quickly can fade quickly.
With exercise, people have a tendency to start too fast then burn out. Here is Hussain discussing that very problem on a popular radio program. With diet there is such a thing as being too strict too fast, and it tends to backfire with you giving up altogether if you don't see immediate results.
Most importantly, whatever your fitness goals, try to find a way to work on them while still having fun. If your New Year's Resolution makes you feel a sense of dread then you are unlikely to stay the course. But if you can find an enjoyable way of eating better and staying active, you'll come to see the work involved as a source of pleasure.