Those who can do – 583 times, in Mark McGwire's case.
But those who achieve at the level McGwire did as a player don't often end up doing what he is doing now – rising early every day, disappearing into the batting cages at Camelback Ranch to teach the next generation of hitters what he learned as one of the most feared sluggers in baseball history.

Thirteen major league teams, including the Dodgers, have gone to a two-coach approach for hitters. Of the 43 current hitting coaches, there are more who never played in the majors (nine) than there are those who made multiple All-Star teams as McGwire did. Seven more had fewer than 100 hits in their major league career. Only four (besides McGwire) made more than two All-Star teams and none approached McGwire's 12 selections in 16 seasons.
Only Harold Baines (assistant hitting coach for the Chicago White Sox) came within 200 of McGwire's home run total – he hit 199 fewer than McGwire's 583 – or exceeded McGwire's career total of 1,414 RBI.
Great hitters, it seems, don't make great hitting coaches.
"You hear that – but there's always one that breaks that mold," Dodgers outfielder Carl Crawford said.
"I don't believe that," said infielder Nick Punto, who also worked with McGwire in St. Louis. "Some great hitters were great with talent and talent alone. Big Mac was a great hitter. But he was also a heady, into-the-game hitter. He knows patterns, understands situations."
Not always.
McGwire said he "basically played the first six years of my career on just physical ability. I didn't really understand what the game was about. ... Then somewhere, somehow I had a major roadblock and had to make changes."
That roadblock was a miserable 1991 season that saw him bat just .201 with 22 home runs, the lowest full-season total of his career. During the offseason, the Oakland A's hired Doug Rader (fresh off his firing as manager of the Angels) as the new hitting coach on Manager Tony La Russa's staff. Rader began working with McGwire that winter and "the light bulb just went on," McGwire said.