As stylist for NBC's reality hit "The Biggest Loser," Liza Whitcraft has the skinny on looking good at any size. And like the show's dreaded weigh-in, her job has its highs and lows.

This season she's had the satisfying challenge of dressing Alison Sweeney after the host had her second child in May. "Every week, I had to put her in a smaller pair of jeans," says Whitcraft. "She's a size 4 now. She looks amazing."

She's also had the experience of putting contestants who were recently morbidly obese into size 6 dresses. "I have to admit, that's a fun day at the office."

Are the newly svelte shy about wearing something form-fitting after years of wearing baggy clothes to hide their girth?

"Hell no," says Whitcraft. "We usually pick big personalities for "The Biggest Loser" and they just want to go out there and show off. If anything, I have to keep them out of things that are skin tight."

Still, the stylist is charged with making all the show's contestants look trim for the season finale -- even if they're not down to their ideal weight. For that task, there is a lot of hidden help involved. Think : Spanx, elaborate bra fittings and compression T-shirts.

Here, Liza Whitcraft shares some of her behind-the-scenes slimming tricks with StyleLIst.

For curvy petites such as host Alison Sweeney:

• Dark, boot cut jeans are your best bet. "Avoid boyfriend styles, anything tight or skinny," says Whitcraft. "Right now, for Alison, I am loving Joe's Jeans in the Petite Provocateur style. The leg break is perfect on a petite women. It adds so much height."
• Heels rule. If you can wear a two-inch heel and be comfy, "Do it."
• Wrap dresses are key. "I put Alison in a lot of Diane von Furstenberg. They are terribly flattering on her, but the truth is she's beautiful. She looks good in just about everything and can wear almost any color."
• Great fitting T-shirts are key to a casual look, especially ones that don't cover too much, making you look heavier on top. "Avoid crew necks. Wear a V-neck whenever you can." She favors tees by Vince and James Perse for Sweeney.

Slimming tips for ladies of all shapes and sizes:

• Just wear black. "It's a cliche that is true. It is soooooo slimming. It is all I would wear if I wanted to look 10 pounds lighter," says Whitcraft. "Biggest Loser" contestants don't wear a lot of it because "it doesn't read well on TV." Instead, she uses rich dark plums, golds and navy to give her clients some warmth and a trimmer appearance. "Not so boring, but almost just as slimming."
• Know your slimming designers. Her contestants wear lots of DVF, Rebecca Taylor, Catherine Malandrino and Tadashi. The latter designer, "Is probably my favorite for these ladies. Very flattering." She is a fan of Tory Burch and California-based designer Karen Miller.
• Accentuate your waist. "You will see a lot of wrap dresses and belts in the finale. You cinch that waist and it is going to take pounds off in a second."
• Wear some kind of a sleeve. "I had a contestant get mad at me last season because I tried to put her in a shrug for the finale, but if you are large or have lost a lot of weight your arms are not so great. I think a little coverage is a good thing."
• Undergarments rule: Whitcraft is a fan of Spanx, Yummy Tummies camisoles and is the queen of bra fittings. "I've had contestants try on 50 bras before I found the right one. You want the perfect lift and style for the dress. And the right bra takes off pounds, too."
• Rock those heels. "Wear four inches if you can. Elongate that leg. You 'shed pounds' by adding height."

Let's not forget about the guys:

Whitcraft says the NBC producers like the male contestants to appear at the finale in dress pants and shirts. When she can, she likes to outfit the men in jeans and a snug sweater to show off their new, improved bodies.
"They need help too," she says.

"When you have lost that much weight, there is a lot of loose skin. It's true of the men and women. Some of our contestants do go on and have plastic surgery to tighten the skin."

So for the finale, it is not unusual for her to slip a snug compression shirt or controlling briefs under their clothes.

You can catch all of her wardrobe magic on "The Biggest Loser," which airs Tuesdays at 8/7c on NBC.

The show's popular "Makeover Episode," which includes an intervention by "Project Runway's" Tim Gunn and hair stylist Tabatha Coffey of Bravo's "Tabatha's Salon Takeover" airs on Nov. 17.