By Valentine’s Day, enticed by myriad chocolate treats, many Americans find that their exercise and diet resolutions have begun to slide.

Why is it so hard to make these changes for good — and how can one get back on track?

“People tend to be on a post-holiday New Year’s high, and thus are less realistic about what they can achieve,” a New York-based life and career coach, Sharon Good, said. “We set these huge, overwhelming goals and think we can achieve them in one leap.”

For those who have abandoned their New Year’s resolutions, Ms. Good recommends easing back into a workout regimen: “Start going to the gym once a week, and once you’ve achieved that goal, move up to twice a week. Soon you’ll be where you want to, without having gone from 0 to 60 in one swift movement.”

After all, real change doesn’t happen overnight. It takes 21 days to adopt a new habit, after which “it becomes second nature,” Ms. Good said.

The owner of Inform Fitness Studios in New York, Adam Zickerman, said that the primary reason that individuals don’t achieve their fitness-related goals is that they fail to find the programs best tailored for their unique body-shaping goals.

“People start too gung ho, take on too much too soon, burn out, get hurt, and turn their lifestyle upside down all at once,” he said. “Their workout simply doesn’t fit into their lifestyle.”

About 15 years ago, Mr. Zickerman stopped a too-strenuous weight-training regimen that left him feeling lethargic and suffering from joint pains. “When you’re training regularly, you should feel great,” he said. “I knew something wasn’t working.”

So he adopted a slower-paced regimen, which ultimately yielded better results, he said. “Lo and behold, I felt better, looked better, and my pain was gone,” Mr. Zickerman, who went on to author “Power of 10: The Once-a-Week Slow Motion Fitness Revolution” (Quill), said.

Inform Fitness, which focuses on slow-motion weight training, is based in Midtown and has branches on Long Island, in Pennsylvania, and in California. The secret to that workout: Each repetition is “10 seconds up, 10 seconds down.”

A celebrity trainer and star of Bravo’s reality series “Work Out,” Jackie Warner, said that resolutions often fall through because people place too much emphasis on cardiovascular workouts and not enough on weight-bearing exercises. “Women, especially, tend to neglect weight training, and it is a very essential element in exercise for weight loss, she said.

Ms. Warner also suggests increasing water consumption to about three liters a day. Doing so can help burn calories and has anti-aging benefits, too. “Your skin will naturally clear up, your pores will tighten, and it literally turns back the clock,” she said.

In addition, Ms. Warner recommends a complete pantry purge. “Junk food, snack food, candy, refined sugars, white pastas, and bread — it all has to go,” she said. “You can’t keep an addiction in the house and then be angry with yourself for having it.”

She insists that, once you condition your body to healthy eating, all those resolution-thwarting cravings will dissipate.

So long, heart-shaped chocolates!