Beginning on January 1, 2020, with the introduction of the World Handicap System, the USGA has added to or modified existing protections agains inflating one's handicap, or "sandbagging."
Please refer to the Rules of Handicapping for the World Handicap System at USGA.org.
Note: The information below applied prior to the introduction of the World Handicap System on January 1, 2020.
There's an excellent article in pdf form titled, "Fairway Felons," by David Barrett in the Chicago District Golfer magazine. He begins by stating,
"Sandbagging. No one likes to admit that it happens, but we all know it does. Here’s how to spot it and what your club should already be doing to get rid of it once and for all."
According to Barrett "virtually all" forms of sandbagging fall into three main types:
Barrett points out that the USGA Handicap System includes a number of buffers against golf sandbaggers. Chief among them is the protection against those golfers that mysteriously - or not - step up their game for tournaments. Throwing out the 10 best handicap differentials from the most recent 20 scores also mitigates against sandbagging, as does multiplying the average by 96%. Barrett goes on to offer eight guidelines for clubs to use to protect the integrity of the handicap system from golf sandbaggers. Read the full "Fairway Felon" article.