1. Monitor your diet. Obtain enough calcium and vitamin D, preferably from the foods you eat. While dairy products may be the richest sources of calcium, an increasing number of foods, such as orange juice, are calcium-fortified now. Fruits, vegetables, and grains provide other minerals that are essential for bone health, such as magnesium and phosphorus.
2. Maintain adequate weight. This is particularly important for women. Menstrual periods often stop in women who are underweight, probably due to a poor diet or excessive exercise. Estrogen levels are therefore too low to support bone growth.
3. Quit smoking; limit alcohol intake. Smoking and too much alcohol intake decrease bone mass.
4. Include weight-bearing exercises in your workout. Regular weight-bearing exercise like walking, dancing, or step aerobics can benefit your bones. Include strength training in your exercise routine.
5. Discuss with your doctor about your risk factors. Certain medical conditions, such as celiac disease, and certain medications (steroids and others) can increase your odds of developing osteoporosis. Talk with your doctor to develop a prevention strategy that accounts for these factors.

(Source: Harvard)