Removing sweat stains from bras can be a bit tricky. You want a treatment that is effective on stains, which often means it's a bit harsh, but still gentle so it doesn't ruin a delicate fabric. In this situation, you can certainly use some of the gentler methods already mentioned, such as diluted white vinegar or baking soda; but there are also a couple of different approaches that work particularly well on bras.
Begin by looking at the bra's fabric content. Acetate and rayon are pretty delicate, while nylon, polyester and Spandex are relatively sturdy.
For the not-so-delicate fabrics, you'll wash as you usually do with the addition of an enzyme pre-treatment. Enzyme pre-wash products contain protease, which breaks down proteins. You can find products that are used just for the pre-soak purpose, or you can grab a liquid detergent that says it contains enzymes. In either case, pre-treat according to the instructions on the container.
For bras composed of extra-delicate fibers, try a different process: Wet the stain with water, sponge on a wet spotter (a mixture of water, glycerin and dishwasher detergent -- see How to Make Wet and Dry Spotters) and apply a couple of drops of ammonia to the stain. This concoction should absorb the stain so it washes away when you rinse it out. The trick here is to really pull it out: After saturating the stain itself, soak an absorbent pad (or a good paper towel) in the solution and let it sit on the stain, keeping the pad and the fabric moist until the stain is completely drawn into the pad.
Removing sweat stains from bras and shirts can be tough, but it's typically doable -- it just may take trying more than one of these approaches. Preventing sweat stains, on the other hand, is often a lot simpler: Wear an undershirt to soak up the sweat before it reaches your nice shirt, and, if these stains are a big problem for you, avoid antiperspirants that contain an aluminum compound. Aluminum is great at preventing sweat, but it can also react with any sweat that does come through and make stains even worse.
Or, invest in a really great jacket. At least you won't mind covering up.
For more laundry tips, check out the links on the next page.