Hair twisting, nail biting, nose picking, and shirt gnawing are just a few of the annoying habits 6-year-olds develop. Your child isn't out to irk you. Such habits are a way of coping with stress. Nagging to stop is actually counterproductive. It only draws negative attention to the habit, making your child more nervous and attached to it.
Instead, talk about it in a more casual way. Identify the habit and offer rational reasons for stopping, ones that are clear and understandable to a 6-year-old. "Picking your nails can give you an infection that could hurt." Or "You've been growing out your hair for a long time now. But if you keep chewing on it, we may have to cut it because it's not a polite thing to do."
Enlist your child's help in coming up with ways to stop. Suggest a secret wink or hand motion to signal him when he's chewing on his shirt, for example. Offer positive reinforcement: If he can keep his shirt out of his mouth for a month, you'll buy him that sports jersey he's been begging for.
Most of these unpleasant habits disappear as kids get older and learn other ways to handle anxiety. Peer pressure will help, too. Not too many kids let their classmates get away with picking their noses.
Your child's longer attention span allows him to extend a play theme over a few days or across multiple playdates now. Try to set aside a place in your house for your child to leave out things he's working on, such as art projects, forts or playhouses, or block cities.