IT must have been my shoes.

Before I headed over to a brunch on the Upper East Side to audition as an escort for the upcoming International Debutante’s Ball, my father gave me one piece of advice.

“Shine your shoes,” he said. “They always look at your shoes.”

The bachelors’ brunch is where young men of means (I was there undercover) go in hope of being picked as escorts at the December coming-out party for society girls from all over the world.

Young, single men of a certain background stand in a line to be looked over by ladies who were selected by an international group of blue bloods, counts, princesses, earls and even a marquis, whatever that is.

Each debutante gets to pick a civilian and a military escort – who carries a state or country flag behind her during a “presentation” ceremony.

When I arrived at the chandeliered Au Bar for the 46th such cattle call, men named Ash and Ned and Lawrence, in bow ties and military uniforms, were going toe-to-toe with women named Whitney and Courtney dressed in silk and satin. It was like a well-catered Whit Stillman casting call.

I got there late, so I was last in line, next to a Dartmouth freshman named Trip who was eagerly explaining to moms and daughters how he’s “dry pledging” his college fraternity – which means swallowing live goldfish and getting tossed from the frat-house roof into a trash dumpster instead of chugging endless beers.

The stakes are high at get-togethers like this. The ball – a charity for the Soldiers’, Sailors’, Marines’ and Airmen’s Club, to be held Dec. 29 at the Waldorf-Astoria – is where young Social Register types are formally introduced to international society. Engraved invitations to the very best parties flow like Veuve Cliquot from affairs like this.

It’s not the most elite such event, but plenty of socially (and financially) appropriate women are willing to shell out $6,500 for the privilege of attending. (Invited men go for free.)

There are certainly boldfaced names: Amelia Eisenhower Mahon, great-granddaughter of the late President Dwight D. Eisenhower, is representing the United States this year.

Well-received debs are often invited on the summer ball tour, whisked from one ritzy European gala to the next. It all culminates in the super-fancy Viennese Opera Ball, the ultimate coming-out party.

“People see the ball as elitist and oppressive,” said Meade Jennings Morrison of Pennsylvania. “It’s not. It’s really great to get dressed up in a gown and have men act like gentlemen around you.

“In today’s world, you can go to Wall Street and act like a man and be treated equally, but sometimes it’s just nice to get all dressed up and be treated like a lady. Even the wild party guys that come . . . act like perfect gentlemen at the ball. It’s tradition and it’s fun.”

The right date can mean everything.

As the afternoon progressed, I tried to ooze mannered charm, smiling and running for drinks. I flattered the moms and aunts, and flirted gently with the young ladies.

I recited my own meager pedigree – originally from Manhasset, N.Y., I went to the near-prestigious South Kent school in Connecticut, was graduated from Boston College and now reside on the fringe of Brooklyn Heights, (OK, in a fourth-floor walk-up). To compensate, I tossed out bon mots like whiffle balls and offered up witty rejoinders whenever appropriate.

A couple of the young women appeared mildly interested in me, so I pressed my case – politely, of course.

I counseled a few on post-collegiate careers. I offered to escort one young fashion student to the offices of Cosmopolitan magazine, where a friend of mine works.

Throughout the afternoon, one young lady in a pretty pink sweater kept coming up to stand near me as I chatted.

“This is it,” I thought. “I’m about to be snagged.”

She and her friend kept looking at me, and then whispering to each other. I edged closer to try and listen. I made myself look as approachable as possible. They kept backing off.

I went to the bar, where I ran into West Pointer Johnny Ulsamer, 20, who was nursing a Coke by himself. Ulsamer hadn’t been picked yet, but he didn’t seem to mind.

“I felt like I was in the frozen-food section of the supermarket,” he said, smiling. “They paw at you.”

As the brunch broke up and the ladies headed for their coats, I stood in a corner, a bit disappointed. I glanced around for the girl in the pink sweater, who was nowhere to be found.

Then I caught a glimpse of my feet – in spit-polished shoes, complete with scuff marks on the side. Chalk another one up for the old man.