What an excellent question! If at all possible, it's definitely better to attempt to prevent osteoporosis than to treat it. A healthy diet is a top priority, since being too thin is known to be a risk factor for the condition. Adequate amounts of calcium (1000 mg for adults under 50, and 1200 mg for those over 50) and Vitamin D are essential, take a look here for more details: http://www.healthcentral.com/osteoporosis/c/9487/9301/vitamin/

Regular weight-bearing exercise is also important -- this can include walking and dancing but unfortunately biking and swimming aren't much help to bones. Lastly, avoid smoking and excessive drinking.

We can't change our genes, so if despite your best efforts you find you lose bone mass as you age, don't be discouraged. By taking these steps you will certainly help keep the problem from being less severe than it otherwise might be. Take care and good luck!