Best ways to stay healthy and have a strong immune system?

Lifestyle & behavoir. Just like you already know: Maintain proper nutrition, add vitamins if your diet is poor. Adequate sleep and exercise. Stress reduction. Avoid infections with proper hand hygiene and vaccinations. Seasonally avoid touching your eyes, nose and mouth when around people, ie shopping, working, gatherings and travel.
Diet, exercise,happy. A healthy diet is essential for good health- the basis should be fruits & veggies. See A quality multivitamin is also wise. Minimize exposure to environmental toxins. Being happy is essential! This often involves doing work you enjoy, helping others, having good relationships & a spiritual connection. Try reducing stress- meditation helps. Also stay physically active.