When it comes to panties, notes Rebecca Aspan, the author of "The Lingerie Handbook," a cotton crotch is a must for breathability. You should also match the rise of the panties to the rise of clothing: high-rise bikinis with low-rise jeans are a fashion faux pas.

Shown are "Hanky Panky" thongs from French Dressing in Beacon Hill. They're one size fit all. Thongs are so popular the store offers a Thong of the Month Club.