Luann Lee doesn't waste time. Get a good look as she gallops past, because Miss January never occupies a space for long. A Valley girl with a Wall Street turn of mind, Luann was graduated from Thousand Oaks High School a year early (of course), then turned the full force of her attentions to the challenge of making it big, Yuppie style. She wanted to be a singer, but not a poverty-stricken one. So she got down to business first, becoming, at 21, one of the youngest Metropolitan Life insurance agents in the company's 114-year history. She left the Met to be a national sales rep for one marketing firm, then became national marketing director for another. At which point she put money in the bank and went back to singing. Sounds easy, doesn't it? Now, at 25, she's lighting up Las Vegas' Maxim Hotel as the newest of Playboy's Girls of Rock & Roll. And she's starring - VCR owners, rejoice - in the fourth Playboy Video Centerfold, available now if you hurry. Luann makes the most of her investments, both of time and of money. Guess what she bought last month? "A hundred and twelve thousand pounds of sugar," says the fledgling commodities trader. "Sugar futures have to go up only a few pennies for me to parlay that into a chunk of money. Cotton may be next." This is the consummate late-Eighties woman. "I'm pragmatic," Luann says. "I didn't want to starve to be a singer, so I followed the business route. Now I'm singing again. Things have worked out wonderfully. Working hard when it's worktime and playing when it's playtime - that's the way to be a Yuppie." She is actually a Yguppie - a young, gorgeous, urban professional - with two bits of advice for Playboy readers. The first: When you're hung over, try an Agatha Christie novel and popcorn. "This is my theory," she says. "The mystery keeps you engrossed, while the popcorn gets to your stomach and absorbs everything." The second: Bid those sugar futures up.