Mr. Boothman... is Dale Carnegie for a rushed era. THE BIG CITY By JOHN TIERNEY   He had written a book titled "How to Make People Like You in 90 Seconds or Less," but how could anyone in New York take that seriously? What did a guy born in England and now living in Toronto know from our mean streets?   The author, Nicholas Boothman, rose to the challenge and showed up Wednesday morning at Grand Central Terminal. Give him 90 seconds, he said, and he would charm anyone we encountered in Midtown. If he could make friends here . . .   He was directed to the most unapproachable-looking person in Grand Central: a tall, very pretty blond woman standing alone in the center of the station, apparently peeved at being kept waiting. She was dressed in black from head to toe, arms crossed protectively in front of her chest, face set in a stern don't-even-think-of-hitting-on-me expression.   Mr. Boothman strolled over to her. He had dressed to give off an air of authority (with a double- breasted blue blazer) as well as approachability (red loafers). His hands were above the waist with the palms turned toward her so she could see he wasn't carrying anything dangerous. He faced her with his heart pointing at her heart ("There is magic in this," he writes). He made sure to smile and make eye contact.   "Could I ask you a question?" he said, and she nodded. "When you meet someone for the first time, how can you tell whether or not to trust them?"   "Intuition," she said.   "How does that work for you?" he asked. Mr. Boothman loves questions that start with "how" — an "opening-up word," he calls it, because it invites long, relaxed responses instead of the curt answers you get with questions that start with "Do you" or "Are you."   The woman was already smiling back at him, 15 seconds into the encounter. She launched into an animated explanation of what kind of people earned her trust — people like Mr. Boothman, obviously. Mr. Boothman, who is 54, calls himself a practitioner of neuro linguistic programming, a field he switched to after a career in fashion and advertising photography. He'd noticed during photo shoots that some people instantly got along with everyone else, and some people never did. "We decide within two seconds how we're going to react to someone," he said. "We're hard- wired to make snap judgments based on people's body language."   Mr. Boothman's book, which was published last November by Workman and is being translated into seven languages, is Dale Carnegie for a rushed era — a short manual on body language and conversational tricks for those who don't have much time to win friends and influence people. The techniques looked hokey on the page, but not when Mr. Boothman put them into practice Wednesday morning.   In short order he brought smiles to the faces of four other commuters at Grand Central before moving on to the 44th Street headquarters of the New York Yacht Club. He didn't quite succeed in talking himself into the club's inner sanctum, but he did have a long friendly chat with the quartermaster at the front desk after asking, "How do you like working here?"   After a stop at the Carnegie Deli, where he made friends with two waiters and the manager, he descended into the Columbus Circle subway station and fearlessly accepted a new mission: get sightseeing advice from the clerk at the token booth. As passers-by looked on in amazement, the clerk leaned forward to him, turned up the volume of the intercom and smiled as she discussed the relative merits of the World Trade Center versus the Empire State Building.   The final challenge was mission impossible: get shopping advice from a clerk at a Duane Reade drugstore. Mr. Boothman picked up a bottle of ibuprofen, made a slight humming sound, and asked a clerk, "How can you tell whether to take this or Tylenol?" Soon the clerk was poring over the fine print of boxes on the shelves. After five minutes of discourse on pain relievers, the clerk offered to take Mr. Boothman back to the pharmacist for more advice.   Mr. Boothman declined and headed back out on to the formerly mean streets of New York. What had gone wrong? Why had New Yorkers been so nice all day? Was the city's reputation doomed?   "Everyone we met today could have been difficult," Mr. Boothman said. "But people are willing to be cooperative if you can get them to feel comfortable, and that's why everyone we met today was great. New Yorkers are like presents under the Christmas tree — open them up and they're full of pleasant surprises."   Christmas presents? We may never live this down.