Just got a clean bill of health from my doctor :) but he said I need to take precautions to avoid getting atrophic vaginitis. What can I do?

Local estrogen. Depending on your age and past history, a very small amount of local estrogen cream in your vagina once a week should easily prevent atrophic vaginitis. Ask your gynecologist.
Topical estrogen. If you have no contraindications, then using topical estrogen (cream, vaginal tablet, vaginal ring) is the best preventive. Vaginal moisturizers, (not lubricants) can be helpful. Maintaining regular sexual activity also helps keep blood flowing to the area & mantains vaginal health.