1. Limit antibiotic use.
In other words, don't take them for the flu (they won't work anyway). "Antibiotics are like spraying weed killer on your backyard," says anthropologist Jeff Leach. "You knock everything out, and the invading plants come back first." When you do take antibiotics, follow them with a probiotic supplement. Look for those that contain lactobacillus and bifidobacterium strains, require refrigeration, and have an expiration date and a logo that signifies some sort of independent certification from organizations like USP and NSF. You might also want to use that hand sanitizer more sparingly – after a train ride but not before every meal (regular soap and water is better).