A friend of mine recently complained that her new James Perse top, much like this James Perse Slouch Fleece Top ($165), was ruined by yellow pit stains. Sucks. Pit stains are ugly, but they're not the end of the world. The first plan of attack is to try to prevent them. Should they succeed in making their way onto your shirt, there are ways to obliterate them. Mua-ha-ha . . .
To learn how to fight pit stains, read more.
Sweat alone doesn't cause armpit stains. The chemicals from your antiperspirant are also to blame. The best thing to do is to switch from an antiperspirant to strictly a deodorant. I did. If you must use an antiperspirant, try an intense one that will more actively stop sweat, like Klima ($25), which you apply once every five days.
To remove pit stains, spray Shout Laundry Stain Remover ($5) on the problem area and let it sit for five minutes. Then take a brush and scrub the area until the stains have been removed. You can also use RID White-Wash Dye Powder ($3) to get yellow stains out. A more obvious solution is to wear loose-fitting tops so that the arm holes don't stick to your armpit. Lastly, avoid wearing a jacket or cardigan during sweaty times; the extra layer will force your top to stick to your armpit. Am I missing anything?