8)Hormone Replacement Therapy (HRT) has been shown to be one way to reduce the incidence of osteoporosis in post-menopausal women, however recent clinical trials have indicated there are dangerous increased risks of heart disease, stroke and various cancers associated with HRT. What is the single most important factor in reducing the incidence of osteoporosis in older women and when do women need to start being concerned about this?

I think the best method to help prevent osteoporosis is exercise. Exercise strengthens the bones in men and women.

Also there are good medications on the market to help prevent this condition:

If the prevention and treatment of osteoporosis is the only issue under consideration, then bisphosphonates such as alendronate, ibandronate, or risedronate are more effective than menopausal hormone therapy in preventing osteoporotic fractures, and less likely to be associated with substantial adverse effects. So far, bisphosphonates are the most effective category or prescription medications for treating postmenopausal osteoporosis.

hope this helps..