New Year's resolutions are stupid for 2 reasons:
First, they only work with the will, the conscious mind, which is maybe 10% of you.
Second, they are normally said, "This year, I'm going to..." Saying "I'm going to" puts whatever follows into the future, giving you the subliminal message not to do that now, but sometime in the future. The only time we can do anything is in the present, so telling yourself to do something in the future is a way of telling yourself not to do it.
What you should do instead is affirmation. An affirmation is a positive, present tense statement of what you want as if it already existed. It must be something you can control -- affirming that someone else will do something is useless. A simple delcarative statement works best. Affirmations are generally best when they are 7 - 11 words, according research cited by Paul Pearsall in his book, "Wishing Well".
So instead of saying, "This year, I'm going to get in shape", say, "I now enjoy exercising daily", which will have the effect of getting you in shape. The more specific you can be, the better, so "I now enjoy walking 3 miles a day and lifting weights 3 times a week" would be even better.