What's the worst thing that can happen: Although it's disgusting to watch your child's finger go from nose to mouth, eating boogers won't make him sick. But picking his nose might: The mucous membranes lining your child's nostrils are a prime entry point for infections.
How to make it stop: Redirection is usually effective. "Give him a toy or a book -- anything to occupy his hands," says Michal Nissenbaum, Ph.D., a psychologist at the Developmental Disabilities Center at the University of Kansas Medical Center, in Kansas City. Explain to a preschooler that nose-picking spreads germs. Also carry tissues, and hand him one the instant he reaches for his nose.
Bonus tip: Reduce itching by keeping his nostrils moist. Dab them with petroleum jelly, or use saline drops several times a day. During the winter (or if you live in a dry climate), a cool-mist humidifier in his bedroom will prevent mucous membranes from drying out overnight.