The last thing a girl wants on prom night is to show up in another girl’s dress. This year, Mercer Island High School senior Ludi Besenovsky developed a plan to keep that from happening. At least, she is trying to.

An active member of the online social networking site, Facebook, Besenovsky has set up a system whereby MIHS girls can post photos of their prom gowns online. The purpose is to prevent other girls from buying the same dress, and the method is simple. As a social networking site, Facebook allows members to start “groups” in which they can share comments and photos online. All Besenovsky had to do was start such a group and, with the help of some friends, invite her classmates to upload pictures of their prom dresses.

“I invited about 200 girls — basically the whole grade — to join. I tried to include everyone,” said the senior, who learned of the idea from friends at other Eastside schools.

Two weeks later, the Facebook project has taken off, with more than 50 dresses already pictured.

Unfortunately for Besenovsky, her good-hearted idea helped everyone but herself.

“Literally two hours after I started the group and posted a picture of my dress, another girl said, ‘I have the same dress,’” Besenovsky said.

The senior is taking her ironic luck with humor and grace. After all, it’s not so much the dress that makes the girl as the girl who makes the dress.

“It’s not the hugest deal,” Besenovsky said. “She’s pretty short and I’m really tall, so I didn’t really mind.”

When it comes to prom, trends come and go every year. For the Class of 2008, short is the new long.

According to Christina Tan, a sales associate at the Bellevue Square Macy’s, the days of long and flowing prom gowns are over.

“Long dresses are out,” said Tan, who works in the juniors department of Macy’s. “It’s definitely the short dresses that are popular, especially the ones that are kind of bubbly or balloon at the bottom.”

The 50-plus dresses posted on Besenovsky’s Facebook site support this observation. Most of the dresses are fitted and short. Dozens are strapless and, because this year’s prom theme is “A Black and White Affair,” there are fewer pastel colors than in previous years. Besenovsky will be one of many girls in black.

The dress she chose — along with her double — is a long, backless, BCBG gown.

“Usually, when I think of prom, I think of a long, formal dress. My whole mentality is you may as well wear a long dress,” said the senior.

Long dresses and short, the past month at Macy’s has been a flurry of satin, polyester and silk; gowns strewn across fitting room doors and piled over metal clothing racks.

“The fitting room is freaking out,” Tan said. “The girls try on 20 dresses and often don’t buy any. We can’t always put things back into the right place due to staff constraints.”

The department store was especially busy last week during its “20%-off Friends and Family Sale.” Many parents, who finance their teenager’s prom experience — from shoes to limousine rentals — are thankful for any discount they can get, especially when prom dresses can run up to $500.

Besenovsky is borrowing her gown — priced at $400 — from a friend.

“Quite a few girls are borrowing dresses,” the 18-year-old said. “My dress ran at $400, so I didn’t feel like spending that kind of money on it.”

Macy’s offers dresses on the more affordable side.

“We have low prices. The average cost is $35 because we have a sale going on,” Tan said.

The juniors department also sells dresses in the $100 to $150 range. Most of these are longer and more elegant in style — another reason for going short this season.

But when it comes to searching for that special, one-of-a-kind prom dress — the one that no other girl has claimed — Islanders often have to look beyond Bellevue Square.

“I know a lot of people who have found dresses online that aren’t that well known. Or they go to small boutiques in downtown Seattle or Kirkland,” Besenovsky said.

Others buy their dresses overseas, during a summer trip to Paris or while visiting colleges in New York.

Whatever the case, this year’s MIHS prom ought to be a kaleidoscope of originality. Thanks, in part, to Besenovsky’s last-minute idea.

“I just think it’s fun to see what dresses others are wearing,” she said. “And hopefully it will help prevent girls from buying the same dress.”