Kids with allergies tend to pick their noses the most because a heavy flow of mucus can dry and crust inside the nose, giving them a "something's up there" feeling. Nose picking can cause nose bleeds and also spreads germs. Try these tips to break this bad habit:
Address any allergies.
Have your child drink plenty of fluids to avoid the build-up of dried mucus.
Keep nails trimmed so that dirt doesn't build up behind them.
Encourage hand washing.
Keep plenty of tissues available and encourage nose blowing and nose wiping.
If all else fails, teach your child that this is a private activity and not to gross out other people by doing it in public.
DO NOT publicly call out your child for picking his nose.