The key to sticking with an exercise program is choosing activities you enjoy, and mixing it up when you start to feel bored with your routine. Keeping your regimen varied also helps work out your entire body instead of just a few muscle groups. Here are a few to get you started.

Strength training

The best option to start may be to hire a certified personal trainer for three or four sessions to develop a plan for you and show you how to properly use the equipment. You can use weight machines, free weights, or resistance equipment like specially made rubber bands, and you can strength train at a health club or at home.

Strength training videos that show you how to use common household items such as food cans and water bottles can help save expenditures on weights or other fancy equipment. In any case, if you don't use the proper form, you can injure yourself, so you do need to learn how to use the equipment, whether it's from a personal trainer, a video or a book. Be sure any video or book you use is up to date by looking at the date it was published as some once-popular strength training exercises have been found to be potentially harmful.

Strength training is important to women of all ages. In young women, it can set the stage for a lifetime of stronger bones. In women over age 30, it can help slow or reverse the natural process of the degeneration of your muscles. And studies have shown that older women who strength train not only maintain bone density but also are at a much lower risk of hip fractures, due in part to the improvement in dynamic balance that often accompanies stronger muscles.

Aerobic exercise

The options are many and varied. Some of the more popular choices include the following:

Brisk walking is the most popular aerobic exercise among women and is appropriate for women of all ages. Walking at a swift pace burns almost as many calories as running or jogging for the same distance, and poses less risk of injury. If you are a beginning walker, choose a level surface. Gradually increase your pace until you can do one mile in about 15 minutes. To intensify the exercise, add hills and varied terrain to your course. You can also use hand weights of one to three pounds, but avoid ankle weights as they can cause injury.

Jogging burns more calories in less time and is as simple and convenient as walking, but it is too strenuous for some and may cause joint injuries. If you are a beginner, alternate walking and jogging for the first three or four weeks. Then gradually increase the jogging portion until you can comfortably run for the entire workout. Remember not to exceed your target heart rate; the talk test may be the best way to quickly measure your exertion level.

Aerobics classes or home videos offer variety, music and choreography, and some women prefer the extra motivation an instructor provides. Start with beginner classes or videos, and watch the instructor carefully for proper foot placement and body alignment to avoid injury, especially to your knees. There are a variety of types of aerobics classes, including:

Step classes incorporate a low bench that allows you to step up and down while performing various moves.

Boxing classes and Tae Bo have become a craze in some parts of the country. Boxing classes consist of aerobic moves combined with boxing moves such as punching and footwork. Tae Bo adds martial arts moves, including karate-type punches and kicks, to the mix. The feet and upper body move for most of the class, providing a total-body workout.

Slide classes involve a special mat and booties that slip over your shoes and allow you to slide back and forth on the mat. Great for toning the lower body but should be avoided by those with knee injuries.

Interval classes combine step or floor aerobics with weight training using hand-held weights or special rubber bands.

Toning/sculpting classes incorporate floor aerobics with a concentration on one or more specific body part with isometric exercises.

High-impact classes incorporate moves such as jumping, running and hopping, and are not recommended for women with joint problems in the lower extremities.

Low-impact classes incorporate moves where one foot is always on the floor. They are not necessarily low-intensity exercises, though.

Spinning is an exciting aerobic exercise developed in the 1980s. Participants use a specifically designed stationary bike, and the instructor leads the class on an imaginary ride accompanied by energizing music. During an average 45-minute class, you can burn 400 to 500 calories. Be sure to talk with the instructor before your first class to go over the type of clothing you might need (padded shorts), your target heart rate, and your physical limitations.

Swimming is an ideal exercise for women with physical limitations such as musculoskeletal problems and asthma and for women who are pregnant. However, swimming does not raise the heart rate quite as much as other aerobic exercises because humans are equipped with a reflex that causes the heart to slow down when immersed in water. For swimming, use a heart rate target of 75 percent of the maximum minus 12 beats per minute. It is also not recommended as a primary activity for losing weight because the body tends to conserve body fat as insulation in cold environments. For those whose only option is swimming, however, it is certainly better than remaining inactive.

Flexibility training

Don't skip these exercises, because they are beneficial in helping prevent cramps, stiffness and injuries. They also ensure a wide range of motion, particularly important as women age. Two flexibility/stretching regimes are popular enough now that you should be able to find a class for either that fits your needs and schedule:

T'ai chi, an ancient Chinese practice, is becoming popular for older adults. T'ai chi incorporates slow, graceful movements with relaxation and breathing techniques. It is said to improve strength, flexibility, balance, coordination and posture, and is recommended by the National Institute on Aging because it may reduce older adults' risks of falling. The Arthritis Foundation calls it the ideal exercise for arthritis sufferers. Traditionally performed on land, t'ai chi can also be done in chest-deep water for added resistance and support.

Yoga has been practiced for more than 2,000 years around the world, and about six million Americans practice yoga. Yoga increases flexibility, strength, balance and range of motion. It also reduces stress and increases feelings of well-being. Everyone from high-powered executives to stay-at-home moms to people coping with illness or injuries can practice yoga. A typical yoga class involves breathing, warm up, yoga postures that consist of specific ways of stretching and moving the body, and relaxation and visualization. Be sure to find a qualified, certified yoga teacher and begin slowly.

You can buy a video to show you how to do stretching exercises in the privacy of your own home, or you can have a personal trainer at a gym show you how to incorporate the exercises after your warm-down period.

Special considerations: exercising when pregnant

Exercising when you're pregnant can help you achieve better posture, less back pain, less stress, better digestion, more energy, and less "postpartum belly." It can also prevent or control gestational diabetes. If you've exercised throughout your pregnancy, you will be rewarded with increased strength, flexibility, and stamina during labor and delivery, as well as a faster recovery.

Be sure to consult with your OB/GYN about your exercise routine. If you were already active before becoming pregnant, you should be able to continue, within reason. If you are new to exercise, be sure to start slowly and do not overdo. Low-intensity or low-impact cardiovascular exercise such as walking, swimming, low-impact aerobics classes, or special exercise classes for pregnant women are best. You can engage in these activities three to four times per week for about 30 minutes per session. Ask your OB/GYN about a target heart rate; keeping it below 140 beats per minute is recommended. It's critical that you keep your body cool and well hydrated (drink lots of water) during exercise. Don't forget to warm up and cool down.

Strength training during pregnancy can also be beneficial in building stamina and strengthening muscles and bones. Use lighter weights or resistance because heavier weights may increase your chances of injury. Remember to breathe normally. Keep these pointers in mind:

• don't do exercises performed while lying on your back after 20 weeks
• avoid deep knee bends, full sit-ups, double leg raises, and straight-leg toe touches (your ligaments are more prone to injury during pregnancy)
• because overheating can be dangerous to your baby, don't exercise in hot, humid weather or wear excessive clothing
• always drink plenty of liquids and stop and consult your healthcare professional if any unusual symptoms appear, including pain, bleeding, dizziness, shortness of breath, irregular heartbeat, or difficulty walking.

Special considerations: women with chronic conditions

Today, exercise is often recommended as a management strategy for many chronic medical conditions. Of course, a thorough discussion of exercise with your healthcare professional is imperative prior to beginning any kind of program.

For example, for women who suffer from osteoporosis, a bone disease that causes bones to thin and weaken, exercise is highly recommended. A carefully designed exercise program can help protect your bones and retard development of the disease. Weight training, in particular, helps counter the effects of osteoporosis by stimulating bone formation. Avoid heavy weights, and work up to two sets of 15 to 20 repetitions each with lighter weights. Walking, jogging, and aerobics classes also help build bones.

However, bicycling and swimming don't stimulate bone formation. Flexibility exercises enhance your posture and increase your balance, making you less susceptible to dangerous falls.

Exercise also is extremely helpful if you have diabetes. Diabetics who are physically active have fewer diabetic complications. Exercise can lower your blood sugar level, helping eliminate the need for insulin. The American Diabetes Association recommends a combination of aerobic activity, strength training, and stretching exercising three to five times per week for 20 to 40 minutes each time. Your healthcare professional will need to oversee the design of your fitness program.

Always check your blood sugar level prior to exercise; if it's lower than 70 mg/dl or you are exercising more than one hour after a meal, you may need to decrease your insulin or have a light snack beforehand to avoid a reaction. Always have a fast acting sugar source with you in case you do have a reaction, and be sure to wear a medical alert identification bracelet or necklace. There are several other precautions you need to be aware of, so be sure to consult your healthcare professional first.

For women at risk of developing heart disease - if you have a family history of heart disease, are overweight, smoke, or have high cholesterol, diabetes or blood pressure, for example - exercise is crucial. In fact, according to the American Heart Association, lack of physical activity itself is now clearly shown to be a risk factor for cardiovascular disease, the number one killer in America.

Studies have shown that people who are physically inactive are from 1.5 to 2.4 times more likely to develop heart disease - a risk as great as high cholesterol, high blood pressure and cigarette smoking. Even low-to-moderate intensity activities such as pleasure walking, climbing stairs, gardening, yard work, moderate-to-heavy housework or dancing, when done for as little as 30 minutes a day, can bring benefits.

More vigorous aerobic activities such as brisk walking, running, swimming, bicycling, roller skating and jumping rope - done three or four times a week for 30 to 60 minutes - are best for improving the fitness of the heart and lungs. If you already have heart disease, you can exercise safely as long as you work out under medical supervision and carefully monitor warning symptoms.

Check with your local hospital or university for monitored cardiac rehabilitation exercise programs. Strenuous physical exertion is never recommended for people who suffer from heart failure, unstable angina, chest pain, significant aortic valve disease, or aortic aneurysm. Even some of these conditions, however, such as heart failure, might benefit from mild or moderate exercise under controlled situations.

Exercise also is beneficial for and can help control obesity, high cholesterol, high blood pressure, back pain, and the symptoms of some neurological and emotional disorders. It also has been shown to help prevent certain types of cancer.