NEW YORK (AP) — There was a list at the door, but the beautifully dressed guest in the chic, red-soled Christian Louboutin shoes wasn't on it. Still, she insisted she was a friend of the host. Not wanting to offend, the staffer at the door waved her in.

And when the woman proceeded to drink herself silly at the jewellery-store party on Rodeo Drive in Beverly Hills a few weeks ago, resting her body on glass cases and telling a cute male waiter she wanted to bear his children, it soon became clear she wasn't a beautifully dressed guest, but a beautifully dressed party crasher.

"She was wearing Louboutins!" marvelled the embarrassed staffer at the door. As she'd just learned the hard way, party crashing is all about looking the part.

Michaele and Tareq Salahi maintain they weren't crashing when they found their way into the White House state dinner last week, he in a tux, she in a fetching red sari. The White House begs to differ. However this mesmerising and alarming story turns out, though, event planners will tell you that party crashing is a time-honoured tradition.

And a flourishing one, thanks partly to the way news of exclusive events gets out these days, says Alexandra Malloy, who heads a public relations and marketing business in Los Angeles and was present at the Rodeo Drive party.

"It used to be the public heard about an event after it happened," says Malloy. "But now, bloggers are invited, and they'll blog before, during and after. They're tweeting and Facebooking on the spot. People can just show up and try to get in."

And when celebrities are due to attend – many of them get paid for doing so, either in cash or merchandise – it simply adds fuel to the fire. "We're all so interested in celebrities now. There's probably a direct correlation between that and an increase in party crashing," Malloy says.

That means increased pressure on event planners to keep interlopers at bay. "You really want to protect the integrity of an event," says Leslie Stevens, a partner at the communications firm of LaForce+Stevens, which puts on at least two events a week in New York City. "So you really have to keep these people out."

For Stevens and her crew, who keep photos of a few well-known crashers just like restaurants keep photos of food critics, that means being vigilant even before the event takes place.

Because the really sneaky ones will try to get on the guest list by calling to RSVP – even when they weren't invited in the first place. Doesn't work with their own name? They may call back under an assumed name, someone a little famous, but not too.

"You can't call and say you're Matt Lauer," says Stevens, referring to the host of NBC's 'Today'. "But maybe you'll say you're an editor somewhere."

At the door, there are telltale signs, event planners say: Crashers get haughty when questioned. They don't make eye contact. They try to bypass the check-in line.

And often, they don't check a coat – too cumbersome if you need to make a quick exit.

What motivates them? After all, many people are too tired or busy to hang out even where they're wanted. These people, needless to say, are unlikely to become party crashers.

But people have their reasons. Some want to move up on the social ladder.

Others do it for the chance to network professionally. Remember the ambitious secretary played by Melanie Griffith in 'Working Girl', who crashed a wedding reception with the help of dishy Harrison Ford to meet a key executive? Fictional, but still.

And some just have a yen for living dangerously. "There's just some jazz to getting into a place you're not invited," says Stevens, the New York executive. "It becomes a game."

Required: a major dose of chutzpah. Take Stevens' dinner companion years ago, at a black-tie affair at Manhattan's Union Club. He had a nice tux on. He was engaging and charming in conversation.

But soon the table mates, who knew each other, started asking: "Who IS this guy?" Turns out the organisers had no idea either. The guy had scoped out a table with an empty seat, and plopped himself down.

"It's one thing to crash a cocktail party," notes Stevens. "It's another to seat yourself at a black-tie dinner." The man didn't make it to the main course.

Was he satisfied nonetheless with the thrill? That's how Tamsin Lonsdale felt years ago, crashing parties in her youth back in Britain. "It was exciting," says Lonsdale, now 31. "But I think most people grow out of that as they get older."

In any case, Lonsdale has no need to crash parties now: She's the founder of The Supper Club, a highly exclusive moving dinner party with 1,000 members worldwide.

Lonsdale puts on 15 events of varying sizes per month, mainly in New York, Los Angeles or London. Members pay an annual fee, from $2,000, which will get you local events only, to $5,000 for super high-end dinners, perhaps accessed by private jet.

Needless to say, exclusivity depends on keeping crashers out. "We have to be all the more vigilant and careful," says Lonsdale, estimating she encounters crashing attempts three times a month. "I always have a very charming doorman out front," she says. Firm, but polite.

One might think people would feel shame being where they're not wanted. Lonsdale thinks some crashers think they WILL be wanted, once they display their charms and looks. "In Los Angeles, people think it's not about having their name on a list – it's about looking good," she says. (And at some clubs, it surely is.) There are even a few crashers who have gained national notoriety, like Jerry Berliant, who manages to crash exclusive parties and events across the globe, or fellow Chicagoan and sports Dion Rich, crasher of Super Bowls, the Kentucky Derby and the World Series, and subject of a book, 'Confessions of the World's Greatest Gate Crasher'.