Start each exercise with a weight you can lift comfortably 10 to 15 times. After your muscles and joints are toughened up, you can start to increase the resistance until the exercise is somewhat demanding. This challenges your muscles and joints to become stronger. But don't challenge them so much that you get injured. In general, if you are performing 15 repetitions of the exercise with ease, it is time to increase the resistance. After the 4 to 6 week toughening-up period, your muscles should feel tired after your workout. Move the weight through the entire range of motion. Lift and lower slowly and smoothly. If an exercise is performed quickly, the momentum of your movement does much of the work instead of your muscles. You are also more likely to get injured. Do not "lock" your knees or elbows when extending your arms or legs, as this stresses the joint. Exhale as you lift the weight, inhale as you lower it. Keep your breathing even and smooth. Holding your breath can raise your blood pressure. Stop if you feel any pain during your workout. Reduce the amount you are lifting, and be sure you are doing the exercise correctly. A mild burning sensation in the working muscles is OK, but joint pain is not. Some muscle soreness for a day or two after your workout is normal. Sore joints mean you have overdone it. Reduce the resistance or number of repetitions you are performing until the joint pain has been gone for at least two weeks.

Perform a balanced workout.

Each muscle group has an opposing group; work both. For example, the quadriceps muscles on the front of your thigh extend the leg (straighten the knee), while the hamstrings flex it (bend the knee). If only one of the groups is strengthened, the imbalance can lead to injury. If you are unfamiliar with the major muscle groups, ask a trainer for some assistance in designing your program to be sure it is balanced.

Work large muscle groups first.

If smaller muscle groups, like those in the arm, are tired from lifting, then you won't be able to perform the exercises for chest or shoulders at an adequate level, since these large muscle groups require heavier resistances.

Cool down after your workout.

Stretching at this time is particularly effective, since muscles are very warm. Stretching after your workout will help increase flexibility, an essential component of muscular fitness.