—Don't let cold weather keep you from being active—that's a surefire route to winter weight gain. Exercise keeps you healthy, trim and happy, and it's a great antidote to the gray-weather doldrums.

With the right layers of clothing, walking in a winter wonderland can be fun. But in bad weather, head over to the mall for your laps or look for a nearby indoor track.

Also, find opportunities to move at work. The American College of Sports Medicine suggests running an office contest to see who can take the stairs the most in a month—great cardio that also strengthens your legs and core muscles.

Winter is a great time to maximize that gym membership and stay social while staying fit. Try different classes to keep it interesting, then focus on mastering the one you like best. If you've always wanted to expand your in-home gym, now might be the perfect time for a new piece of equipment.

To get exercise through fun activities, explore winter sports in your area. With a little instruction, you can enjoy cross-country skiing or snowshoeing on park trails. Or consider joining an indoor sports league through a local rec center that offers adult programs.

Whenever you're exercising outdoors, dress the part by wearing multiple layers that you can peel off as you heat up to avoid getting drenched by sweat. Wear a hat to help retain body heat and be sure to protect all extremities—fingers, toes, ears and nose.

And always warm up fully before you begin a strenuous workout and cool down afterward.