I think excessive nose blowing can invite infections because a dry nose lacks the mucus which acts as a protective barrier. Nose blowing in moderation is fine, though, in my experience.

Nose picking can also lead to infection, because you're essentially inoculating yourself with whatever pathogens happen to be on your finger.

Aside from avoiding these, the things that have made the most dramatic difference for me is taking Vitamin D and getting flu shots.

Now I virtually never get sick, whereas in the past I'd pretty regularly get multiple colds and occasionally the flu pretty much every year.