Hillary Clinton needs to put a smile on her face. Donald Trump needs to avoid saying anything at all about her face. Those are two of the tips from insiders for the major party candidates as they head toward their first, potentially pivotal debate. Among the others: They need to be careful. The public knows them unusually well, so if they suddenly seem to be undergoing personality shifts, they'll stumble. Clinton and Trump are scheduled to debate for the first time on Sept. 26. They will face off again on Oct. 9 and 19. These are the key takeaways from 2016 campaign debates gone by:

Don't get too personal

It's one thing to challenge a rival's public record, but quite another to insult them personally.

Trump's lowest debate moment came a year ago. He had criticized rival Carly Fiorina's face before the debate, and at the debate, Fiorina hit back hard, "Women all over this country heard very clearly what Mr. Trump said," Fiorina said icily. Trump backed off, saying Fiorina has "a beautiful face."

Fiorina's response "wasn't thought of in advance, tested with a focus group or written by Hollywood elites — it was Carly being Carly," recalled Fred Sadler, her campaign manager. It gave her a brief boost into the GOP top tier, though she soon faded.

Seize the agenda

Trump proved himself a master of controlling the debate and keeping opponents guessing what he'd say next. "One of his strengths is his spontaneity," said Jeff Kaufmann, the chairman of the Iowa Republican Party.

Clinton must be careful not to fall into the trap of spending too much time reacting. She should heed the lesson of Jeb Bush, a former governor of Florida, who tried to match Trump's insults at a February debate in South Carolina. Trump easily won the state a week later. Bush finished a distant fourth and was out of the race.

The lesson, said Dan Gerstein, a veteran Democratic political strategist, is that Clinton should quickly try to present herself as the true change agent. Don't try to go one-on-one with Trump's quips or insults, he said, because whether he's correct or not, many people see the Washington establishment as villains.

Instead, Gerstein said, Clinton should stick to a central theme: that she's been promoting change for years while he has no record. "She has to keep asking, 'What have you done?' " said Gerstein.

Don't get locked into your script

Sen. Marco Rubio, R-Fla., was pretty much done after Chris Christie, the governor of New Jersey, scolded him repeatedly at a February debate for his "memorized 25-second speech." Clinton has a tendency to repeat her points, former House Speaker Newt Gingrich, a Trump supporter, said, so Trump has to be ready to disrupt her rhythm. "She is very prepared for a controlled environment," Gingrich said.

Make people like you

"These debates are ultimately about feelings. You want people to come away with a good feeling about you," said Bob Mulholland, a Democratic consultant.

Sen. Ted Cruz, R-Texas, found that while he had a command of issues and a solid ideology, he couldn't match Trump's appeal. "Trump is willing to do what other candidates have rarely done," said Saul Anuzis, a Cruz adviser. "You don't know what to expect."