Forget New Year's; start a summer workout instead

"People are most active during spring and summer and least active during the winter. In (the) southern United States, activity levels can actually drop during summer because it is too hot and humid," she said. "The seasonal effects are driven by outdoor activity. The weather is warmer and the days are longer, making activity outdoors more enjoyable and safer. Seeing others being active outside is also a motivator."

There are some benefits to exercising outdoors, Parr said. The sunlight can offer you vitamin D and may even boost your mood.

"Activity outdoors leads to enhanced feelings of energy and diminished fatigue, anxiety, anger and sadness compared to similar activity conducted indoors," Parr said.

Take advantage of this increase in daylight with outdoor activities, such as hiking, biking or even running on the nearest beach or lakefront, said Tara Romeo, assistant sports performance director at the Professional Athletic Performance Center in New York.

"There are many outdoor options that can take you out of a gym and take advantage of what's around your area," she said.

Since there is more daylight in the summer, it might be easier to schedule time to exercise with a workout partner. Also, some people tend to have more flexible schedules in the summer months.

Numerous studies show that having the social support of a significant other or friend is associated with better maintaining a workout routine (PDF), exerting yourself more during the workout and feeling calmer (PDF).

"Being physically active with friends and family is motivating and fun. Regular exercise that is enjoyable and built into your daily routine will have lasting power," Heisz said.

"My 4-year-old and I stay active during the summer by riding our scooters in the park," she said. "This is something fun and active we can do together."

Though people might see faster fitness results in the summer, Heisz offered some advice for keeping up those same results in the winter.

"I'm an avid runner, and I find it easier to make excuses to keep from my training in the winter months. That's when I need to draw on my self-regulation the most," she said. "For me, the solution is to sign up for a race. This motivates me to keep up with my training; otherwise, I'll have a difficult time on race day."

Romeo suggested trying a variety of indoor exercise regimens to stay motivated to work out during the winter months.

"The best advice to stay motivated outside the summer months is to change it up. Join a kickboxing gym or learn how to lift weights if you do not know how," Romeo said.

"There are so many possibilities out there. You don't need to be confined to the same equipment all year long. If you are looking to peak for summer season, my advice is to give yourself at least three months, depending on how far off the wagon you are, for your body to adjust and see results."

Parr, the associate professor in South Carolina, agrees.

"Winter is a good time to try new types of exercise, like yoga, group exercise classes or weight training in the gym," Parr said.

"With limited time, exploring shorter exercise programs like high-intensity interval training (HIIT) can be a great way to get in shape in just a few minutes," he added. "The important thing is to make including exercise in your day a priority. Aside from maintaining and improving fitness, keeping active can help you prevent winter weight gain."

Indeed, Romeo said that the key to seeing and keeping fitness results is to keep it up.

"Your body needs to be taken care of year-round," Romeo said. "It's a full-time job but one that is very rewarding."

As long as you stay active throughout the year, you're on the right track, Parr said.

"Really," he said, "there is no 'wrong' time to start making exercise and other health changes. People should pick a time that they think will work for them."