Call it the latest in how to break away from a Washington party when the boss is holding a press conference:

"If I don't get back I won't get a bite to eat," apologized White House chief of staff Donald Regan. Then, more to the point perhaps, he said, "I've got to be up at the president's at 7:30."

White House Communications Director Patrick Buchanan dropped in with the inside skinny on getting the president ready to meet the press.

"His back is up, he's on his game and he's all set. He had two hours of briefing yesterday," Buchanan said, "but we cut it to an hour and a half today because we thought he was so prepared. We feel if he's going well, has his answers down and he's rolling, we should stop it."

So went the big talk last night at the fall garden party of the Decatur House Council of the National Trust for Historic Preservation. Regan, accompanied by his wife, Ann, was guest of honor, sharing the receiving line with Rear Adm. John D. H. Kane Jr., council chairman, and the National Trust's Alan Boyd.

Regan played his role faultlessly, at least for half an hour, by greeting 200 such old-line Washingtonians as Ethel Garrett, Tom Clagett, Gerard Smith and Clarence Dodge, who paid $50 apiece towards the $1.3 million construction costs of restoring the 167-year-old house on Lafayette Square.

And Pat Buchanan? Did he buy his ticket? someone kidded. "Oh, yes, big contributor," Buchanan said. "I used to be a big contributor when I was making money."

At 7, with a hour and a half left to go in the party, Regan headed for the White House to escort President Reagan to his first press conference in three months.

Asked whether the president would have another session next month, Buchanan said, "You may rely on him. We're going to keep to the one-a-month schedule. And we'll do others -- regional briefings and all those things. He likes the mini-press conferences, too, like he did at his desk after the South Africa thing. So we're experimenting with a lot of forms."

Regan missed Secretary of State George Shultz by five minutes but connected briefly with Agriculture Secretary John Block, who managed to press home a point on administration farm policy. "He ran some ideas he has by me," Regan said after huddling with Block. "It's quite a problem; we're very sympathetic."

Block, looking thinner than usual ("maybe a pound," he shrugged), tried to sound optimistic in describing an earlier meeting he and Regan had with Senate Majority Leader Robert Dole of Kansas, the chairman of the Senate Agriculture Committee, North Carolina Sen. Jesse Helms, and House Minority Leader Robert Michel of Illinois.

"They're going to help us try to get our policies through. We know the president's policies are right but we're not there yet," Block said.

It was Susan Baker, looking triumphant after her morning appearance on behalf of Parents Music Resource Center on "Good Morning America," whom George Shultz headed for when he arrived.

"You were a smash!" Shultz said.

"Oh, George!" she said.

"I'm just quoting your husband Treasury Secretary James Baker ," Shultz assured her. "We were at a breakfast meeting and all the sudden he said 'Whoop! I have to go watch television.' When he came back we couldn't get him to talk about anything else."

"Listen, dirty words get lots of press," said Susan Baker of her work with the group, which is trying to get the recording industry to label records with explicit lyrics. "If you want to make news, start quoting dirty lyrics."

"Did you see what Jane Fonda said?" Decatur House Council member Sally Chapoton interrupted.

"Well, I heard that she said things have gotten so bad that even censorship is needed," Baker answered.

"She said pornography's going to be the end of the world," Chapoton continued.

Asked Shultz: "It's going to be what?"