I'm doing the basic precautions, but they should just be daily life now especially if you live in a dense urban area

Get sunlight everyday, if not vitamin d pills

When out in public, wear a face mask, not only is it polite, it helps with pollution.

If you have a runny nose, a cough, chest congestion, anything that feels less than 100%, you absolutely should be wearing a face mask in public. Even if you're the only one doing it, it's just part of the social contract

On the train, if somebody is coughing, I offer them a new face mask, they always take it and put it on after looking around sheepishly, cuz they know it's the right thing to do. So I just carry extra face masks

Wash your hands before touching your mouth, eyes etc. It takes a bit of getting used to to not touch your face in public, but that should just be basic hygiene now

Have a healthy level of cardiovascular exercise, it helps your immune system tremendously. 30 minutes at least every week, it could be walking, anything to get your heart moving

Ensure you're getting enough protein

A multivitamin doesn't hurt, though it can be just wasteful, but if you're not sure about your diet why not

I take zinc supplements, but they are also rumored to help with your immune system

Probably most importantly, like extremely importantly, make sure you don't have diabetes, or insulin resistance, those tremendously hurt your immune system. These are the broad metabolic issues that society is suffering under, and they really make use susceptible to communicable diseases.