When President Obama boarded Marine One for his first flight as commander-in-chief, he decided to break from tradition. Departing from the South Lawn of the White House for a trip to Williamsburg, Va., the new president extended his hand to a U.S. Marine standing in salute at the base of the helicopter's steps.

The unscripted gesture forced the Marine to break his stance and shake the president's hand before he returned to his posed position of hand at brow.

Like his predecessors, Obama must master the presidential protocol that has long ruled the White House. From military salutes to the presidential march to the customs by which foreign visitors are welcomed, every commander-in-chief must respect the traditions that have come to define the office.

"Protocol is diplomacy," said Nancy Brinker, President George W. Bush's former chief of protocol. "It's about ceremony and custom and making sure the president and his office are represented properly as he travels the world."

And should the president need help, he has a full-time coach like Brinker at his disposal to teach him the rules. The State Department -- which first appointed a full-time protocol officer in 1916 -- established the official Office of the Chief of Protocol in 1928.

The president is briefed on proper military protocol by the White House's military office, which includes representatives from the U.S. Army, Navy, Marines and Air Force.

But learning the proper etiquette doesn't always come easy, said Brinker, who described the formalities as "just patterns you have to get used to."

Bill Clinton was often criticized for his inability to snap a crisp salute -- a gesture Obama appears to have  perfected already. Bush was a stickler for punctuality, while Obama has been teased for his frequent tardiness.

"[Bush] was fastidious about meeting on time. If you were late, you got left," Brinker told FOXNews.com.

She recalled one occasion when Bush almost left Italy without her after she arrived "only a few minutes late" to the Vatican. "No matter how well-planned the day is, anything can happen," Brinker said.

Obama is not the first president to depart from the traditional code of behavior.

In 1981, Ronald Reagan began saluting members of the U.S. military. Such salutes had always required the wearing of a uniform, and no American president -- including former five-star Gen. Dwight Eisenhower -- had ever before returned a salute.It is custom not to salute enlisted personnel or officers of lower rank.

Reagan, who took special pride in setting the new standard, later advised Clinton on how to refine it.

"The president generally has the ability to make decisions on how he would like to conduct business, and certainly there are customs that can be altered," Brinker said.

Obama appears to be doing just that.

"When the president telephones senators, he simply picks up the phone and calls them directly," said American historian Barry H. Landau, who served in the White House protocol office for nine presidencies. "Ordinarily, the chief of staff would do it and say, 'Senator, the president's calling.'"

Landau recalled that Jimmy Carter broke protocol while riding in an elevator with then-protocol chief Shirley Temple Black. When the doors opened, Black turned to Carter and said, "After you, Mr. President," as was customary. But the president refused to walk in front of Black, saying, "as a Southern gentleman, after you," Landau said.

"It was not the protocol and she didn't go, so I finally pushed them both off the elevator," he quipped.

But Landau cautioned that there is some protocol that a president should never breach.

Bush was roundly criticized when he gave German Chancellor Angela Merkel an impromptu back rub during the G-8 summit in 2006.

Former Vice President Nelson Rockefeller once famously touched the back of the Queen of England as she danced at the White House -- an embarrassing incident of "lese majeste," as royalty cannot be touched.

"Protocol is supposed to make people feel comfortable," said Landau.

And with the media's constant presence and the speed at which a gaffe can reach the Internet, presidents need to be ever mindful of proper etiquette, he said.

"The presidents are absolutely more aware that any flub -- or conversely anything done well -- will be noticed immediately," Landau said.