This article is from the Piercing FAQ, by Anne Greenblatt with numerous contributions by others.
Because everyone is built differently, not everyone is anatomically
suited for every piercing. Certain daily activities and sports can
prolong healing or prohibit certain piercings. An experienced piercer
will be able to ascertain if the piercing is likely to be successful.
A piercing should not be attempted if there is not enough tissue to
support the piercing. Piercings made in flat areas often migrate or
reject (grow-out). The optimum area to be pierced is one in which the
piercing will be perpendicular to the tissue, like an earlobe
Wearing jewelry of an appropriate metal, design, and dimensions (gauge
and diameter) is important for a successful piercing. Jewelry that is
too thin in gauge is more easily rejected by the body. Wearing jewelry
that is too thin increases the risk of the piercing being torn if the
jewelry gets caught or pulled. Wearing jewelry that is too thick and
heavy may cause the piercing to migrate or reject. The jewelry must
not be so thick that the strength of the tissue is compromised.
The diameter or length of the jewelry must be chosen carefully. The
jewelry should be chosen after the piercing is measured. Some
piercings tend to swell during the healing period. Wearing jewelry
that is too small in diameter or length will constrict the piercing
and cause the piercing to migrate and scar.
The jewelry should be new and should be polished to a mirror-like
shine, regardless of the type of metal. Used jewelry has minute
scratches which can irritate a new piercing and trap bacteria. Oral
jewelry is often scratched even after only a short time of wear. Oral
jewelry collects plaque which is very difficult to thoroughly remove.