I hope this isn't a topic that's a turn-off. My hope is, rather, that it will serve as helpful advice to my female readers.
Awhile ago, I read something interesting. It was a short piece explaining how, over the winter, dead skin cells build up in the armpit area. The result being that come summer, aka sleeveless season, if you look at your underarms, you'll see a darkened area where those cells are caught up in your deodorant or anti-perspirant. Not too attractive, is it? The article I read claimed that this was a particular concern of blonde-haired women, but I think it probably applies to others, as well.
What's the answer? Fortunately, it's quite simple. Give that area a good cleaning and some good exfoliation. I use soap with "scrubbies," such as a salt bar or something with seeds or pumice. Additionally, a good shower scrub, such as my Shower Glow (I know, a shameless plug) will make this an easy task.
Use it once or a few times, and you'll be ready for tank tops!
Now, if the weather would just cooperate.