It turns out the secret is...
Yes, it turns out to be
exercise. The best thing is to get yourself puffed out on a regular basis!
They did a study, and regular endurance cyclists, many over 80 years old, had the immune systems of 20 year olds...
How exercise in old age prevents the immune system from declining
http://www.bbc.com/news/health-43308729
Unfortunately, lifting the potato chip from the packet to my lips doesn't count as "endurance exercise"...
Best wishes!
Dien