some mundane things that work for me-
cooling off if i am hot, as winter-chilled sweat is not a good thing
bundling up [meaning no uncovered skin, head to toe] if i am cold
multivitamins/multiminerals daily along with a gram of C and some E daily
keeping aerobically fit with an hour or so of daily exercise did the most of all to keep my previously regular winter colds/pneumonias from repeating this year, so far, god-willing.
at the first hint of a chill or sniffle, i take "emercenC" or "airborne" - i can't say it is medicinally a panacea but maybe there is some placebo effect taking place, but it makes me feel better.
going outdoors only when exercising or errands, staying indoors otherwise.
in hot weather when the pollens are airborne, i also stay indoors with air conditioning, as much as possible, as the allergies for me often turned into sinus infections/bronchitis/atypical pneumonia. in any case, i learned to be on the lookout for the first subtle symptoms of unwellness so i could try to nip those in the bud before they got too big to handle without seeing a doc.