Nail biting is related to skin picking and hair pulling. These behaviors are labeled as Obsessive-Compulsive Disorders.
Biting often happens when people are in one of two modes. Some nail biters do it in an automatic way, as if they are in some kind of altered state and not really thinking about what they are doing. It’s often when they are engrossed in some other activity at the same time such as talking on the phone or watching TV, etc. For others, their main activity at the time is the actual picking or biting, and they will usually interrupt their other activities to engage in it.
There is a strong commonality seen in the various purposes behind nail biting, skin picking and hair pulling. At a very basic level, these symptoms satisfy an urge. Many report an uncontrollable compulsion to do them. Pulling, picking or biting also seems to deliver a pleasurable or relaxed sensation.
When sufferers feel stressed, doing these things has a kind of soothing effect on their nervous systems and reduces levels of stimulation. On the other hand, when they are bored or inactive, they seem to provide a needed level of stimulation to the nervous system. This may account for why so many people who dislike doing them find it so hard to give up these behaviors. It simply “feels good” at the time no matter what.
Even if you have more than one of these problems, do not give up on yourself. These symptoms can be overcome if you have the 
motivation. More important is learning to “de-stigmatize” yourself. You are not crazy, helpless, morally weak or totally out of control, even though you may feel like you are at times. Once you realize that you are just a person who simply happens to have a bad habit, you can make some serious progress.
Hypnosis for nail biting consists of three parts.
First, nail biting is a nervous habit. So the first part of the therapy is to teach the nail biter how to be more relaxed at all times. To that end, I have found that traditional hypnosis and Ericksonian hypnosis are usually quite effective. That’s because the very essence of hypnosis is relaxation. The biter can learn to apply several different methods to increase both the relaxed state, as well as his/her overall feeling of relaxation and well-being.
The second part teaches the biter awareness of this unconscious habit. Suggestions are given to the unconscious mind to make the consciousness aware that you are about to bite your nails. Then you get to consciously decide whether or not you are actually going to bite them.
There are also several other powerful hypnosis and NLP techniques. As an example, we can help the client to set up an “Anchor” or trigger so that he/she can momentarily mentally step outside of him/herself. This is an effective method that will effectively relieve the compulsion.
The third part of the hypnotherapy is to eliminate the compulsion to bite. There are hypnotic techniques that actually program the client with a compulsion to not bite.
With this particular behavior, being realistic is very important. First, you have practiced the unwanted behavior hundreds or even thousands of times. It is important to accept that the urge to bite your nails will not be overcome in a few days.
Second, you are fighting the fact that biting your nails feels good, and it offers a lot of short-term satisfaction and either stimulation or soothing feelings. It can take time to overcome, but it will be worth it. My experience has shown a number of techniques to be very effective!