Plexiglas Principle
The Plexiglas Principle, as defined by Bill James, holds that "all things in baseball have a powerful tendency to return to the form which they previously held. If a player's batting average jumps in one year, it will usually decline in the next. If his HR total drops sharply in one season, bet on him to improve it the next."
The fancy statistical term for the Plexiglas Principle is "Regression to the Mean" -- a principle stating that, of related measurements, and selecting those where the first measurement is either extremely high or extremely low, the expected value of the second is closer to the mean than the observed value of the first. The implication is clear: luck plays a part in everything, and luck is, by definition, not a repeatable skill.
What's that in English, you ask? Consider the phenomenon of the "Sophomore Slump." For example, Carmelo Anthony of the NBA's Denver Nuggets had an outstanding rookie season in 2004. It was so outstanding, in fact, that he couldn't possibly be expected to repeat it: in 2005, Anthony's numbers had slightly dropped from his torrid rookie season. The reasons for the "sophomore slump" abound, as sports are all about adjustment and counter-adjustment, but luck-based excellence as a rookie is as good a reason as any.
Of course, not just "sophomores" experience regression to the mean. Any athlete who posts a significant outlier, whether as a rookie (young players are universally not as good as those in their prime seasons), or particularly after their prime years (for most sports, the mid to late twenties), can be expected to perform more in line with their established standards of performance. The trick for sports executives, then, is to determine whether or not a player's play in the previous season was indeed an outlier, or if the player has established a new level of play. Needless to say, it ain't easy. Melvin Mora of the Baltimore Orioles put up a season in 2003, at age 31, that was so far away from his performance in prior seasons that analysts assumed it had to be an outlier... but in 2004, Mora was even better. Mora, then, had truly established a new level of production, though he did regress to his more reasonable 2003 numbers in 2005. Conversely, Kurt Thomas of the New York Knicks significantly ramped up his production in 2001, at an age (29) when players typically start to play more poorly. Sure enough, in the following season Thomas was his old self again, having regressed to the mean of his established level of play. John Hollinger has an alternate name for the law of regression to the mean: the "Fluke Rule." Whatever you call it, though, regression to the mean is a fact of life, and also of sports.
Similar to the Plexiglas Principle is the Johnson Effect, named -- somewhat tongue-in-cheek -- by Bill James for Toronto sportswriter Bryan Johnson, who first brought to James' attention "the tendency of teams that exceed their Pythagorean projection for wins in one season to relapse in the following season."
