For a New Year's resolution that lasts, start now, start simple, and be specific.
Almost half of all Americans make a New Year's resolution each year, research shows.
Most commonly, people aspire to lose weight, exercise more, quit smoking or eat healthier. Many will abandon their resolution by a month or less into the new year.
One barrier is that forming a habit takes time -- about 66 days, on average, at least one study suggests.
"It takes about two to three months for someone to start feeling confident and comfortable with a new habit, but the repetition of the habit can start to become more of a routine after several days" Dr. Beth Frates, president of the American College of Lifestyle Medicine, told ABC News.
When deciding on a resolution, Frates recommends considering the six pillars of health: nutritious diet, physical activity, restorative sleep, elimination of substances (e.g. cigarettes), positive relationships and stress reduction.
"A potential first step is to compare your daily habits to the established national guidelines for these pillars," said Frates. "A person can see where they are in relationship to the guidelines and rate how important each guideline is to their health and wellbeing."
And studies show that simpler habits form faster than complicated ones.
Drinking a glass of water each morning will stick a lot more quickly than doing jumping jacks before work every day, for example.
"But at the same time, you don't want to pick a behavioral goal that is too easy, since that is unlikely to be challenging enough to hold your attention," said Dr. Jennifer Ashton, ABC News chief medical correspondent and author of "The Self Care Solution," a book about creating self-care habits that stick.
Proper planning is also important.
"A reasonable goal for someone is a SMART goal," said Frates, explaining that a "SMART" goal is one that is specific, measurable, achievable, relevant and time-bound.MORE: No. 1 trick to making New Year's resolutions that stick
A goal to "run 4 days a week for 30 minutes" is more specific than "I will run more."
One way to measure success is by marking a calendar or an app with each run.
Achievability of a goal should be based on one's current patterns. For those who currently don't run at all, a goal to "run 2 days per week for 20 minutes" is a smarter initial choice for long-term success.
To keep a goal relevant, consider the "why" behind it, such as "running will make my heart stronger."
A time-bound goal has a deadline for when to reassess success. After four weeks, for instance, a new SMART goal can be made based on the previous month's progress.MORE: Expert tips to stay on track with your New Year's resolutions
"Setting a short-term goal that aligns with your long-term goals will help you to stay on track," said Frates.
The good news: One missed day is OK. Research shows that missing one day does not reduce the chance of habit formation.
The key is to keep going.
"The process of getting through the lapse and successfully getting back on track usually happens within the first two months," said Frates.
So starting now, experts say, is the choice that could boost that New Year's resolution from a pattern to a habit by January's end.
Alicia Zellmer, MD, is a resident physician in Internal Medicine and member of the ABC News Medical Unit.