Protect with sweat pads: You can steer clear of embarrassing sweat stains by affixing a sweat pad to the area of the shirt under your arm on each side. The pad will soak up the sweat and keep your skin dry.
Switch to an antiperspirant: Deodorant does nothing to stop the increase of sweat marks on clothes. Try antiperspirants instead as these will go a long way in keeping you dry in the area.
Go for talcum powder: It will absorb the sweat better before it soaks your clothes. Additionally, it acts as an astringent, meaning that it causes your pores to hamper which may help with sweating. Try to clean your armpit with utmost care regularly while bathing in the morning.
Shave your armpits regularly: It will keep the sweat from accumulating in your armpits by washing away all the bacteria. If you clean that area regularly, it also helps in avoiding unpleasant smell and of course, protects from sweat stains.
Clean it with wipes: Be it wet or dry wipes, both works well to control the sweat. They are easy to carry in the bag and whensoever you get the time to go to the washroom just wipe off your underarms at regular intervals. It keeps you fresh and you will surely be able to breathe easier since you are sweat-free.