in 2002 Norcross, John C., Mrykalo, Marci S., & Blagys, Matthew D. (2002). Auld lang Syne: Success predictors, change processes, and self-reported outcomes of New Year’s resolvers and nonresolvers. Journal of Clinical Psychology, 58(4), 397-405.
found that those who made new year resolutions to make life style changes were more likely to succeed (46%) after 6 months than those who tried to change their behaviour at other times of the year.(4%)
However a lot seems to depend on will power and support
In a longitudinal study of new year resolutions John C. Norcross and Dominic J. Vangarelli (1989) found that from 200 resolvers over a 2 year period only 19% lasted the whole 2 year period. Those who were successful often reported more will power, stimulus control and social support. but 53% of the whole group had at least one slip.
If you lack self control read this article on why it is so difficult to resist temptation.
Gino, F., M. Schweitzer, N. Mead, and D. Ariely. "Unable to Resist Temptation: How Self-control Depletion Promotes Unethical Behavior."
Search for more references to articles on new year resolutions relating to self control, will power and life style changes on pubmed
For advice on creating resolutions that you can keep try the American Psychological Association blog.