Your bones are constantly being rebuilt in a dynamic process involving the removal of old bone through osteoclasts and regeneration of new, healthy bone by osteoblasts. Load-bearing exercise works to build stronger bones by stimulating cells responsible for the synthesis and mineralization of bone (osteoblasts).
Weight-bearing exercise is actually one of the most effective remedies against osteoporosis, because as you put more tension on your muscles it puts more pressure on your bones, which then respond by continuously creating fresh, new bone.
A good weight-bearing exercise to incorporate into your routine (depending on your current level of fitness, of course) is a walking lunge, as it helps build bone density in your hips, even without any additional weights. Running and jumping are also effective, as is weight training. As recently discussed in the New York Times:
"Sprinting and hopping are the most obvious and well-studied examples of high-impact exercises. In one recent study, women ages 25 to 50 who leaped like fleas at least 10 times in a row, twice per day for four months, significantly increased the density of their hipbones.
In another, more elaborate experiment from 2006, women who hopped and also lifted weights improved the density of their spines by about two percent compared to a control group, especially if the weight training targeted both the upper body and the legs. Women whose weight training focused only on the legs did not gain as much density in their spines." ... read more...