In viewing a question about nose-picking, I was considering other habits and how the solutions applied to those habits. What does one do if the habit is hair chewing? Lip chewing? Nail biting? . . . Are there any universal remedies?
Habits exist because they make it easier for us to cope with some situations. If you focus on eliminating the habit, coping will become more difficult. Try to focus on meeting the need that the habit is soothing. For example if a child is hair biting when she is worried, teach her to recognize her body's signs and her mind's signs that she is worried-- including hair biting. You can do this by asking her how she feels, or how her tummy feels and how her head feels etc-- when she is biting her hair. Then, once she can identify these signs, she can use them to trigger a safe, acceptable way of coping, such as deep breathing, talking to a trusted friend, singing, meditating, or exercise. Any time you want to break a habit, think of what you want to replace it with.
Hair chewing, lip chewing, and nail biting typically begin as self-soothing behaviors. So much of How to go about weaning a child from its "Lovey" is relevant here.
In addition, identifying sources of stress is very important. Ask the child what they worry about, and then listen. If the child doesn't articulate what bothers her or him, then try to be observant. If you can identify situations when the child is likely to start the habit, you may be able to help prevent it.
Teaching the child other self comfort measures is very important. Though rare, hair chewing can lead to bezoar production. Aside from cosmetic concerns lip biting and nail biting are generally safe.
Calling attention to the habit will not help.
Occasionally a habit that begins because of anxiety can continue because it is just physically soothing. For this situation, some may be acceptable, and others may be worth trying to desensitize. Exercise, dancing, or other physical play are great ways to cope with physical habits because they shifts the focus of the mind and body away from the habit's specific body part into something more fun and productive.