A strong Immune system will play the prominent role. Excersize regularily. Eat lots of salads. Raw veggies give you 100% of their nutrients. Get at least 20 minutes of sun exposure per day, and if you can't, take a Vitamin D suppliment. Echinacea is only useful when the cold/flu symptoms begin. Taking it when a cold/flu is already in progress will yeild little results. Daily suppliments I take are a) Omega 3-6-9, b)Selenium w-Beta Carotine (boosts immune system) c) Vitamin D suppliment. Also, if possible eat one orange per day. Processed orange juice is useless. Freshly squeezed juice is only good if taken within 1 minute after squeezing. Also, (and this may seem ridiculously academic but..) avoid contact with others who are sneezing, sniffling & slobbering. If you take public transit and someone coughs (etc) get away from them to minimise exposure. Lastly, don't smoke. Just don't do it.