Out of body experiences can be one of the exciting things that can happen to you, but sometimes they can scare the hell out of you. My first OBE did just that because I had no knowledge that such a state of consciousness was possible and I thought that I had nearly died.
Sometimes OBEs happen at an inappropriate time, like during hypnosis or at the dentist's office, while under nitrous oxide. My concern about having an OBE under such situations is that I won't be able to hear the hypnotist's or dentist's instructions since an OBE almost always distorts local audio.
Other times, an OBE may need to be terminated simply because some unpleasant situation is taking place or your physical body may need your consciousness back. I have learned a method to abort an OBE that almost never fails.
We don't know for sure what causes the OBE state of consciousness to occur, but a good theory is this state is produced by some fluke of the reticular activating system (RAS) located in the brain stem (mid-brain). The RAS has several functions. First, it screens our physical senses and alerts the brain to unusual sensory stimulus. This happens even when we are asleep. If the RAS senses something unusual happening, it wakes us up.
The second thing the RAS does is control our state of arousal, that is to say, whether we are awake or asleep. It also puts us into the dream state and paralyzes our muscles in this state, so we won't hurt ourselves while dreaming. Otherwise, we would tend to "act out" our dreams.
Drugs like LSD seem to confuse the RAS into thinking that we are awake and dreaming at the same time, and also causes the RAS to let sensory information through which would normally be screened out. The result is the saturated colors, visions and altered sounds we experience while under LSD.
An OBE is like an LSD experience with one important difference. Under LSD, the muscles are not paralyzed, but during an OBE, they are, just as if in the dream state. This paralysis is probably what causes the "vibrations" that often announce the onset of an OBE.
Understanding all these processes is your first step in gaining full control of effortlessly moving from one state of consciousness to another. A good source of information is the telecourse Understanding Human Behavior. Your community college library may have this series of videos. Be sure to view the lessons on sleep, altered states of consciousness and sensory deprivation. These all relate to OBEs.
Next, you need to condition yourself to be able to move effortlessly to sleep and wakefulness. As you drift off to sleep, think of the number 20. As you wake up, think of the number 1. Learn to associate 20 with sleep and 1 with awake and alert.
Once you have gotten the hang of this, you will be able to effortlessly terminate any unwanted or unpleasant OBE. All you need do to return to your physical body from the OBE state is think of the number 1 and attempt to move a finger or toe of your physical body. The fingers and toes seem to be the least paralyzed, and when you attempt to move them, it seems to snap the RAS back into the normal waking state. Once you learn this process, you will have nothing to fear from an OBE, and you may even get to the point where you enjoy having them instead of being scared to death of them.
There are several good books on the subject of OBEs but there are also a lot of bad ones. Probably the most famous is Robert Monroe's, but many others are just as important. Generally, these books instruct you on how to induce an OBE and what to do once you are "out" of your physical body. As far as health risks go, I wouldn't encourage anyone with a heart condition or other medical problems to try to have an OBE but I think there is little danger for anyone in normal health. As I said, my first OBE scared the hell out of me, but once I read the literature on the subject and practiced a little, I managed to turn a frightening experience into an enlightening one.