There is an article in this months Outside Magazine about bone density, and building strong bones. The article points out that when you sweat you loose calcium, so it is important for athletes to get a lot of calcium.
It also points out that non impact exercise like swimming, walking, or cycling doesn't prompt your body to build strong bones like running, or weight lifting does. Due to sweating swimmers, walkers and cyclists tend to loose bone density the fastest.
The article recommends the average active person gets 1200mg of calcium a day. A glass of skim milk contains about 300mg of calcium, a yogurt around 400mg.
Don't like dairy? You can get your calcium from these sources:
Besides consuming more calcium, you can also catch some rays - the vitamin D from the sun helps you absorb calcium.
Excessive consumption of alcohol, and soda may also cause deterioration of bone density.