Working out at the gym may seem better than battling cold and inclement weather. But indoor exercise quickly loses its charm, especially if you do the same thing every day. "Your body becomes more efficient, so you burn fewer calories. And without variety, boredom can quickly set in," says Richard Cotton, chief exercise physiologist with First Fitness, a personal training organization. Plus, repeatedly stressing the same joints increases your risk of injury. Cotton's suggestions to shake up your routine:

Circuit training: Instead of exercising for 30 minutes on one machine, try 10 minutes on three different machines. Change the order each time, too.

Interval training: If you prefer a certain machine, don't just set it to one speed and go. Adjust the speed and resistance, and push hard for one to three minutes. Lower resistance and recover, then step it up again.

Aerobic/strength training mix: Alternate aerobic exercise with strength sets. For example, try three to five minutes on the treadmill (or bike or stair climber) followed by biceps curls. Aim for at least 20 total minutes of aerobic activity and 8 to 12 different strength exercises.

Try a new class: Check out at least one kickboxing, spinning, or yoga class.