Is there something I could do naturally to prevent osteoporosis, without drugs?

There are several. A diet rich in calcium (dairy, dark green vegetables, fortified foods) and vitamin d (fish, eggs, liver, fortified foods) can help. A little sun exposure stimulates your skin to make vitamin d. Avoid or limit smoking, alcohol, and sodas. Muscle building exercises stimulate bones to take in more calcium. Many prescription meds can cause bone loss; talk to your doctor about these.
Osteoporosis. A healthy diet with the right amount of natural calcium and vitamin d as well as weight bearing exercise (like walking) are the most important things you can do.