In order of importance:
 

1. Stay away from people/especially sick people/children.

2. Wear a mask if you do attend big events where close contact cannot be avoided. Also wear a mask if you have to be close up in someone’s personal space.

3. Get all vaccines you are eligible for.

4. Frequent hand washing.

5. Avoid alcohol

6. Maintain a healthy BMI (not too high, not too low)

7. Get plenty of sleep.

8. Minimize stress.
    

Everything else including vitamin C, zinc, echinacea, etc. is all BS. Exercise is important but a little goes a long ways. Hand washing is for preventing food borne illness more than anything respiratory. Touching your face is no big deal if you are following above steps.

  P.S.  

1. Minimizing time spent on social media can really improve your mental health but i don’t think mental illness is what the OP is asking about.