Why a New Year's Resolution Fails
It seems as if we go through this every year. Come January 1st, people start thinking about what parts of their lives they can improve and hours later they begin an ambitious plan to make themselves better. This is a good thing right? If everyone actually stuck to their resolutions, then yes, this would be a good thing. As quickly as these life changing plans are drawn up, they're abandoned and by the end of January, people are back to their old ways. If you'd like to witness this phenomenon, visit your gym in January and February and notice the traffic spike and quickly fall.
The main reason that a New Year's Resolution will fail is because it's too ambitious. The entire premise of resolutions, that you can change a major part of your life overnight, is a bit wacky. Just as Michael Jordan can't pick up a bat and be a good baseball player without years of practice, you can't snap your fingers and decide to start eating healthy and exercising if for the past few years you've been eating nothing but junk and exercising as much as a snail.
People that go into a resolution expecting to change everything quickly must realize that change doesn't happen on its own. Change takes a lot of work and dedication. If you're not used to eating healthy foods, how do you plan on adjusting to your new diet? If you're not used to exercising 4-5 times per week, how do you plan on getting used to your new routine? The answers to these questions may seem simple but when you start to implement your plan after January 1st, you'll realize that planning change is a lot different than actually changing.
The best way to get around this problem is to set a realistic goal. Making a broad goal such as "eating healthy" or "getting into shape" will end up in failure because of all the work you'll need to put into make such a goal work. Instead, try a narrower approach such as "eat one salad everyday" or "start a daily walk". These goals are a lot easier to accomplish which will give you confidence to make even bigger and more important changes.
New Year's Day can be the starting point for your lifestyle change but it doesn't have to be the end. Don't stop at only one of these 
small goals. These smaller goals should be used to show you that you have the ability to change your life. Once your daily walk and salad have turned into habits, make another few easy goals to reach. Maybe you can add some push ups and sit ups into your daily walking routine and also cut out a few unhealthy foods: candy, soda, potato chips.
Ambitious goals also have a place in the New Year. If you really want to "eat healthy" or "get into shape" then you can. A way to make these big goals a reality is to write up a plan on how you will make the change. Simply wanting to eat healthy or get into shape isn't enough. Start by making a list of what eating healthy requires (examples: reducing your saturated fat and sugar intake, getting more of your fat from unsaturated sources, cutting back on fatty meats, eating more healthy snacks). You can do the same with exercise. Each month, pick two of those items and only concentrate on those. If you practice a new habit for a month, you're much more likely to stick with it.
Putting your plan in writing and displaying it prominently can also give you motivation to stick with your resolution. You can also write a resolution contract with yourself that details what you plan to do and how you plan to do it. Sharing your goals with others is another way you can motivate yourself.