Have you tried using lemons? I never tried using them on my underarms, but I have used lemons on my elbows and knees to lighten them. Before you go to bed each night, mix lemon juice (half a lemon or about a tablespoons worth of the lemon juice) with a teaspoon of olive oil; mix together as well as possible with a toothpick; and rub the mixture into your under arm skin (make sure the area is clean, so that other products are not prohibiting the mixture to absorb into the skin). It could take a few weeks to months of doing every night, depending on how much lighter you want that area to get. There are other natural skin lighteners such as potatoes and aloe vera, but lemons smell the best in my opinion :)
NOTE: Keep in mind that lemons are sensitive to the sun. Never go out in the sun with lemon juice on your skin. Lemon are acidic and will cause your skin to burn and become darker if exposed to the sun.
Hi Crystal, I'm not sure what methods you've been using, but this one has been working for me.
While showering, I use buffing pads (I've found them at Sally's and the 99 Cents Store) on my underarms. I lather soap on my skin and them scrub them clean with the pads. It removes all the leftover deodorant and exfoliates really well. I guess the best part of this method too is that you can use the lemon mixture with it, and get a double punch!
I agree with Crystal about exfoliating, but good old baking soda is a less abrasive way of exfoliating the skin. If you happen to be able to get your hands on an aloe vera plant, you can also break off a leaf and mix the gel with the lemon juice too. Aloe vera and lemon juice would be an ideal combo, but lemon juice with olive oil is more convenient.