So, you've found a perfect holiday dress: The décolletage is deep. The waist is cinched. The back is exposed. The shoulders are bare.

Whoa! Slow your sleigh bells.

You want to make sure that dreamy confection looks just as good on you - even with assets not quite show-off ready - as it does on the hanger.

Enter underpinnings expert Mona Lisa Jackson.

Jackson, owner of the 19-year-old Center City lingerie boutique Coeur, is among the city's most renowned experts in bras, shape wear, and any other spandex-rich secrets that hold women together.

We asked Jackson for tips on how to look our smoothest in the season's dressy ball gowns and sparkling sheaths without having to worry about bra straps or - worse - panty lines.

This dress has two bra-related issues.

"Look at how deep the front of the dress plunges," Jackson said. "You don't want to ruin it with an exposed bra at the breastbone."

The other challenge, Jackson said, is that most of the back is exposed.

So she chose this bra (right) because of the depth in the separation of the cups. The bra also has a mechanism that allows the back to be pulled lower - and therefore to go undetected.

The good news: Because the gown is full, there is no need for shape wear.

Ritratti Sensation Five in One Bra, $95; Invisible thong panty, Ritratti Sensation, $51.

Sheaths - strapless or sleeved, fishtail or straight, ankle or knee-length - are challenging because the close fit shows all imperfections, even the ones barely there.

"It's about eliminating lines," Jackson said. "The smallest of women wear Spanx."

Jackson chose a strapless corset (above) by Le Mystere. The padded cup pushes breasts up, brings the waist in, and cuts down on visible back fat. For extra reinforcement, Jackson suggests a high-waisted Spanx.

Strapless corset, Le Mystere, $98; Hide and Sleek High Waisted Spanx, $58.

The trick to looking put together in this metallic, beauty – off the shoulders underneath the cape - is a strapless bra.

"Because there are capped sleeves on this dress, we think we can wear a basic bra," Jackson said. "If she wears a bra with straps, she'll end up adjusting the dress all night to hide them."

That's no fun.

The other piece to this well-fitting puzzle, Jackson said, is a waist-cincher (right). Yes, they are tricky to snap. Yes, you will want to pull this sucker off the moment you walk in the door from merry-making.

Still, the hourglass shape they create is magic.

Bra, Lou Paris, $69; briefs, Lou Paris, $25; Spanx Boostie Yay waist cincher, $84.

All items courtesy of Coeur, 132 S. 17th St., 215-972-0373,