Although fitness centers offer a wide array of options, it's easy to fall into a narrow rut of using the same treadmill or exercise machine day in and day out. No wonder "boredom" is among the top excuses for not sticking with an exercise program.

"You hear about hitting a plateau - and it happens," said Andrea Sprik, personal trainer with the Avera Fitness Center. Muscles can also get in a "rut," never being challenged beyond a certain level of performance. "As a result, your body is never going to get into top condition," she said. To overcome such roadblocks, Sprik recommends cross-training, which simply means using two or more types of exercise in one workout, or in alternate workouts.

Cross-training is a boredom-buster. But more importantly, no one activity can yield all the potential benefits of exercise, such as cardiovascular fitness, weight loss or enhanced flexibility. For weight loss, it's key to include a weight-bearing exercise, as well as strength training. Weight-bearing exercise - exercise that works against gravity, such as walking, jogging, dancing or stair-climbing - burns proportionately more calories than non weight-bearing exercise, such as swimming or biking. "Weight-lifting raises your metabolism so that you're burning more calories when you're at rest," Sprik said.

Women might shy away from lifting weights, fearing they'll bulk up. Yet for most women, having a low level of testosterone prevents that from happening. "It's true that initially you might gain a couple of pounds, because you're changing that fat mass into muscle mass. Muscle weighs more, but it takes up less space," Sprik said. To keep from over-developing muscles, keep repetitions higher and weight lower.

When choosing specific activities for your exercise program, keep in mind your age, health condition and physical fitness, Sprik advised. A great way to start is walking, either on a treadmill, track or sidewalk. A stroll is different than a vigorous walk, Sprik said. "If you're doing it for cardiovascular improvement or for weight loss, you're going to want to push to get your heart rate up." Treadmills may keep you walking faster, because you set the speed and don't unknowingly slow down. Walking on the street might be harder on the feet and back than walking on an indoor track or treadmill. Vary incline on a treadmill to simulate the different terrain you would cover on the street. When planning your workout, consider the benefits of various types of exercise:

- Swimming improves cardiovascular conditioning, muscle strength, endurance, posture and flexibility. Swimming works shoulder and leg muscles, and it takes the pressure off your joints. "It's also great for the core - for your abs and your back," Sprik said.

- Biking gives the legs a workout, while putting a low level of stress on the joints. A recumbent bike will work the gluts, while an upright bike focuses on the quads.

- Jogging or running, one of the top calorie-burning exercises, is great for your leg muscles. To prevent injury, however, those who are out of condition should begin with walking, and work up to intervals of walking and jogging.

- Elliptical machines provide variety with a number of programs and take pressure off the joints. Different settings, such as resistance and incline, give you a more strenuous workout.

- Stair-climbers provide an intense cardiovascular workout, and are weight-bearing, making them beneficial for weight loss.

- Aerobics add fun to a workout, and at the same time burn a lot of calories. The variety of movement boosts coordination as well.

- Pilates and yoga enhance flexibility and strengthen the core muscles in the abdomen and back. "That strong core is going to help you with everything," Sprik said.

For the best results, work up to 30 minutes of exercise a day, every day. Incorporate weight-lifting for two to three days a week, on alternate days in order to allow your muscles to rest. When exercise begins to become easier, increase the difficulty or intensity. Or, challenge yourself with a new activity. It's true that any exercise is better than no exercise. "But if you want to see results, you do need to push yourself," Sprik said.