I know this is an odd subject, and maybe even embarrassing to some, however my armpits seem to strike up a conversation through my video comments:
I wish I can tell you what it is I do to keep my underarms so clean, but I guess you can say its what I don't do. Surfing the net, I found that most women suffer dark underarms due to harsh deodorant and shaving. I don't use deodorant, nor do I shave. I know, sounds weird, but I guess I was blessed to not have body odor and thick hair! I have very thin, few hairs under my arms that I just leave them. Sounds sick right? haha
If you are suffering from dark underarms there are a few remedies you might want to try. Try rubbing lemon in circular motions after your shower. I hear some people may be allergic, so try a small amount at first. Another tip is papaya soap. I remember papaya soap was sold all over Philippines. Many Filipinos love to have that white skin ;) You should also try exfoliating under the arms every time you shower. You can find loofah's for only $1. Here is an article I found on the web that may be helpful as well.