Get fitted for bras.No, seriously.The Crab left a comment on GFE about how the underwires in her running bras leave welts, which caused me to have a dropped-jaw moment and immediately email her with the words "Dude: Your bras don't fit. Underwires should never leave welts."*And I'm right. Know how I know? Because I went to the local Dillard's and asked for the ancient woman who's worked in lingerie sales for eight hundred and fifty of her nine-hundred-plus years, and I got fitted for a variety of bras. After that, I wandered through the Title Nine catalog and bought several different styles of bra in varying sizes. When they arrived, I tried them on and sent back the ones that didn't feel good. No chafe, no welts, no red marks--even after an hour-and-a-half marathon with Attila.Two years later, I had to do the same thing, because I'd lost fifteen pounds. My chest measurement has increased, but my cup size has decreased. Again, I went to find Methusalette and get re-fitted, and again, I ordered various bras from T9 and tried them on, sending back the ones that weren't pitch-perfect.Why go to all this trouble, you ask? Why not just wear the same 36B you wore in high school?Because bras are just as important as good running or cross-training shoes. They are pieces of equipment, especially the sports bras. You'd never tolerate downhill mountain biking or rollerblading without the proper safety equipment; why not give your rack the same sort of treatment as your head and wrists?So what makes a good sports bra, and why are they so dadratted important?Let me answer the first question second and the second question first.For those of us who are blessed (ha) with ample boobage, the reason for a good sports bra is obvious: running or hopping about with little to no support is painful. There are ligaments in each of your girls called "Cooper's ligaments" which can actually stretch or tear if you bounce around too much. Never mind that that movement is distracting, both to you and to any onlookers. A good sports bra can even enhance your workouts--like if you're doing heavy weightlifting and don't have to worry about smashing your own breasts with a barbell.For those of you who are lucky enough not to have to worry about knocking your eye out while running, there are other issues, like nipple chafing (owie owie owie) and even mastitis (an inflammation of the tissue in the breast) if you're not properly supported. Trust Auntie Jo on this one: back in the day, when I was little more than an A cup, I ended up with a jolly case of mastitis because my running bra was too big and chafed like crazy.Okay, so I've convinced you to wear a good sports bra. Define a "good sports bra", then, you cry. Okay! I will!For smaller-breasted women, a "good sports bra" is usually a compression-type, cotton-blend thing that has hooks in the back or pulls on over the head. It spreads the breast tissue out over the chest wall, thus preventing movement, pain, and embarassment. Some women of medium boobage swear by bras like Champion's, which have integrated underwires. Bras for the small-chested are usually not the works of engineering that bras for the larger-chested are.For women like me, who enter a room in stages, "engineering" is not too strong a word to use. Underwires are pretty much mandatory for women with a C cup and up, with or without compression. I wear bras that are "encapsulating" bras, which means that they hold each breast in a nonelastic cup, thus preventing both bounce and the figure-eight wiggle that comes from running. Usually, sports bras for C cups and up have nonelastic straps over the shoulders and minimal stretch, if any, around the bottom of the cup. There are bras out there, like T9's "Last Resort Bra", that combine both a non-stretch fabric and rows of medieval-looking hooks to compress really big boobs, but I find those awfully restrictive.No matter what size you wear, wicking and coolth are important. Fit, though, is the thing that will make or break your experience.For fit advice, you can go to places like Nordstrom's, Neiman's, or even Intimacy, which is a solely-bra-based chain store with branches in most large cities. I would advise against going to Victoria's Secret--not because they're bad at bra fitting, but because they size their bras so differently that their sizing only works with their bras. (I do love their cotton bras for everyday wear, though. Cheap and they wear like iron.) Or you can do it yourself, or with the help of a friend, following instructions like these Whatever you do, when you go to buy your next workout underpinnings, try on several different styles in sizes that are a little bigger and a little smaller than what you're sized as. Bras vary as much as jeans do in sizing, if not more, and cut and style can make a huge difference.Share your own tips and favorites in the comments, will ya?