From what I understand, magnesium plays an important role in calcium
utilization and retention. Below are some links I found when Googling for
"calcium magnesium bone density".

http://www.jrussellshealth.com/osteo.html
http://www.drrobinmayfield.com/newsletters/newsletter-9.html
http://courses.washington.edu/bonephys/opcalcium.html


Make sure your magnesium intake matches your calcium intake, make sure both
are not too high, and make sure you get enough sun or some other source of
Vitamin D.

-Tad