MSN has a list of stuff you shouldn't wear to the gym. Not for fashion reasons, but for practical reasons.
1. Clothing that's 100% cotton. If you're just lifting weights and not doing much cardio, it probably won't make a difference. But if you work up a good sweat, cotton absorbs it, which can irritate your skin, cause breakouts, and increase friction in areas that are prone to chafing. Which is why synthetic fabrics that wick away moisture are so popular.
1. Anything Baggy. It's a safety hazard, because it's more likely to get snagged on something, or get in the way of what you're doing. And if you're working out with a trainer, baggy clothes make it harder for them to check your form. With that said, you also shouldn't wear anything that's so tight it might mess with your range of motion . . . like pants that aren't stretchy enough.
2. Jewelry. Things that swing around like necklaces and bracelets can distract you. And wedding rings are a bad idea when you're lifting weights, because they can cut your finger, or just get damaged.
3. Boxer Shorts. Especially for things like running or cycling, where there's a lot of rubbing going on. Boxer-briefs are better, but the best option is compression shorts, which have even more support. And shorts made specifically for cycling have extra padding.
4. Low-Impact Sports Bras. Obviously this one depends on how much support you need. But in general, high-impact sports bras with wider straps are better for things like running. Low-impact versions are better for things like yoga.
5. Old Shoes. You probably know this one, but just in case: Working out in old sneakers increases your chances of slipping, and also of damaging your ankles, knees, or hips. And that's not just when you're running, it's even when you're doing something like squats. If you only exercise two days a week, replacing your shoes once a year is probably fine. But if you run every day, you should get new ones every three months.
6. Perfume or Cologne. When your body heats up, it intensifies the smell, which can actually be strong enough to bring on a headache . . . for you, or the person running next to you. (MSN)