The best way to prevent lung cancer is to avoid smoking and to avoid breathing in other people's smoke. If you smoke, quit. While the risk for former smokers remains elevated when compared to a nonsmoker, it continues to fall with each year of smoking cessation. In fact, after quitting for 10 years, an ex-smoker reduces their risk anywhere from 30% to 50%. Just do it!
There is little evidence that eating a healthy diet can prevent lung cancer, although there are many other benefits. There have been many attempts to reduce the risk of lung cancer in current or former smokers by giving them high doses of vitamins or vitamin-like drugs, but none of these trials have worked out favorably. In one study, a nutrient related to vitamin A called beta-carotene actually increased the rate of lung cancer, so it's back to the drawing board!