New Year's resolutions are notoriously difficult to follow through on. An oft-cited 1989 study in the Journal of Substance Abuse found that less than 20 percent of subjects making resolutions had hit their goal two years out. It's not that resolutions are unachievable: It's that there are several common mistakes people make when setting their resolutions. So before making your own pledge this year, take a look at what not to do instead of repeating past mistakes. These are the 15 hardest New Year's resolutions to keep, and why they always fail.