(Hint: It has nothing to do with size.)

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For a garment that’s been worn the world over for at least a century, it seems there’s an awful lot we don’t know about our most intimate accouterment: the bra. Well-quoted statistics suggest more than 80 percent of women are wearing the wrong size. Then there's the notorious struggle to find the perfect strapless style that won’t constantly fall down. So much about bras remains a frustrating mystery. Thankfully, there are people who can help, from professional fitters to experts who will recommend a brand specifically suited to your shape and cup size.

Even with all these helpful hints, though, there’s always something new to learn. I was surprised by a tip I was given by Heather Cvitkovic, director of apparel merchandising at Brooks Running Company—a leading active apparel brand. “Your bra should never celebrate a birthday,” Cvitkovic said. “A sports bra needs to be replaced every six to 12 months.”

We expect more of our sports bras than our regular bras—stretching them further than a normal bra (exercise expands our range of motion) and covering them in sweat, which has corrosive properties that cause the fabric and elastic to break down quicker, even with proper care and cleaning.

To maximize the life of your sports bras, Cvitkovic suggests “having three in your rotation. This will give you multiple options and extend the life of each.” If you're an active woman who hits the gym three times a week, stick to the six-to-12 month rule and keep an eye out for key signs that your bra is past its prime: a stretched band or straps, faded tags, spots that cause your skin to chafe, or a less-than-fresh scent even after being laundered.

And make sure to invest in supportive, well-constructed sports bras right at the outset.

To buy: Brooks Running Moving Comfort Juno Bra, $60; zappos.com.