While the media often portrays the more extreme SM behaviors, the reality is that a lot of SM play never goes beyond a playful spanking. Just as there are ways to reduce the risk in activities such as scuba diving or driving a car, there are ways to reduce the risk and engage in SM behavior safely.
Sane is knowing the difference between fantasy and reality. Fictional accounts of S+M are often distorted for fantasy sake, and are not representative of real situations and relationships.
Sane also distinguishes between mental illness and health. A real distinction between mental illness and health is when a behavior pattern causes problems in a persons personal life.
As is washing your hands until the skin is peeling off, or so frequently that you can not otherwise function is a sign mental illness. S+M, like any other behavior, can be a sign of psychiatric problems. However the VAST majority of its practitioners find that SM enriches and promotes functionality in the other areas of their life. (Unfortunately, there are a few exceptions, and know a few).
Consensual is respecting the limits imposed by each participant at all times.
Consent is the prime ingredient of S+M. One difference between rape and heterosexual intercourse is consent. One difference between violence and S+M is consent. The same behaviors that might be crimes without consent are life-enhancing with consent.
The type and parameters of control are agreed upon by the people involved, and the ongoing consent of all participants is required.
Some practitioners use a safeword, which is a designated word that signals the scene must slow down or stop.