Style 101.(Buyers guide)

Style 101

The Right Fit

The difference between a bikini that looks good and one that should be cast out to sea is fabric, cut, and fit. * No matter what size you are, your bikini should have a high Lycra content. "You want at least 90 percent for support," says Kathleen Egan Mudd, owner of Canyon Beachwear in Santa Monica. Make sure it's four-way stretch Lycra, not Raschel, which has two-way stretch. * A bandeau top is most flattering on a B cup or smaller. "Look for bandeau tops that have a three-and-a-half-inch stay -- a strip of plastic -- under the armpits so they don't scrunch down," Mudd says. "A ring or a square in the middle creates ruching -- and curves." * A triangle top "should cover the breasts completely or show a little cleavage," says Jill Roberts of Jill Roberts boutiques in Los Angeles. * Busty women look best in halter tops. "If it tugs on the neck, try an underwire top with straps," Mudd says. * The most flattering bottom is one with side ties. "Make sure the knots sit at the hip bones -- any lower and the bottom will look too small," Roberts says. * A basic bikini bottom is dowdy if it doesn't dip a little in front and "go narrower in back than you might think," Mudd says. "The common mistake is to cover the entire butt, which only makes it look bigger."

Style 101

The Bikinis

* A knot at the bust draws attention to the cleavage, making the chest look fuller. Chanel stretch

jersey bikini (on reverse), $565, 800-550-0005. * This bandeau top has a square ring and gap at the center to maintain its shape. Lenny polyamide elastane bikini (1), $139, at Breeze, Brooklyn, 718-376-2557. * The classic itsy-bitsy bikini with a triangle top and tie bottom is adjustable, so you won't bulge out of it. Calvin Klein Swimwear Meryl Lycra bikini (2), $108, at Bloomingdale's, N.Y.C., 212-705-2000. * A boy-shorts bottom looks best when ...