To keep from becoming overweight or obese, you should maintain a healthy diet and exercise regularly.
A healthy diet consists of eating nutrient-rich foods that are high in fiber and low in saturated fat, trans fat and salt. This means eating a variety of fruits and vegetables, whole grains (like brown rice, oatmeal, and whole wheat bread), beans and legumes, lean meats, fish and low-fat or no-fat dairy products.
Regular exercise should include at least 30 minutes of aerobic activity most or all days of the week. Strength training is also a great way to avoid gaining excess weight. Muscle burns three times more calories than fat by increasing your resting metabolic rate (the rate at which your body uses up calories when you're not being physically active).
Talk with your doctor to determine a diet and exercise routine that's right for you.