Having enough sleep is the most important thing AFAIK. You should also make sure you get enough vitamins and other micronutrients - vitamin supplements are popu

Having enough sleep is the most important thing AFAIK. You should also make sure you get enough vitamins and other micronutrients - vitamin supplements are popularly overrated as a cure but this doesn't mean they're useless: they don't really cure but their deficit (and you can hardly expect you are not deficient in any) certainly weakens the immune system. Another interesting thing I have recently found out about is l.reutrti (BioGaia Gastrus) yogurt - it boosts respiratory immunity by increasing histamine.