Remove all the bedding from the mattress and make sure the mattress is dry.
Apply a good amount of hydrogen peroxide to the stain. It is best to use a spray attachment so as not to saturate the mattress. Allow the hydrogen peroxide to sit for approximately eight minutes, according to Mrs. Clean.
Blot the area dry using a dry cloth. Do not rub the area but apply enough pressure to bring up any remaining moisture.
Sprinkle a small amount of baking soda over the damp area to help remove moisture and odors associated with sweat stains. Let it sit for several hours or overnight before vacuuming it up.