The right sports bra is essential to a woman's workout.(Knight Ridder Newspapers)

OK, so you've resolved to start working out this year _ again. You've spent all your Christmas cash on those cute stretchy capri pants, a few cool new tops and an awesome new Walkman.

But the most important item women buy is the one that no one else sees.

Next to your shoes, a good, supportive sports bra is key to any workout program. The bra not only makes your routine more comfortable, some doctors say not wearing one can lead to sagging later on.

Lisa Sorrentino, owner and anointed "head boob" at X-Chrom.com, a Chicago-based Web site that carries a variety of sports bras, said _ that more than 70 percent of women are wearing the wrong size bras.

"Women just aren't familiar with how a bra should fit," she said, adding that a lot of women will buy bras in the size they'd like to be rather than the size they actually are.

Troy Sexton sees evidence of ill-fitting bras all the time. The manager of Strictly Running in Columbia, S.C., works the finish line at races and often sees women "flopping around."

While not every female customer is comfortable with Sexton giving them advice, this is a man who really knows bras.