Make a paste of 4 tablespoons of Baking Soda to about 1/3 cup warm water, use less if to watery, use more if not watery enough. (I just eyeball things sorry)
rub the paste onto the armpits, if the stain is really really bad, do this load last so that the Baking Soda can do its magic! Wash as usual.
For really bright whites boil on the stove in Lemon Juice and Hang Outside to dry. I don't know why all of my Grandmamas tips work, but they do. I do this to my Teenage Sons socks. (Because they are so gross sometimes I think I could gag!) (He does do his own laundry, I steal his socks while he is at school and try to get them white) :0)
The joys of being a mommy