Ok, a co-worker was surprised at how muscular my arms are the other day, and I told her the exercises I do. These banish (horrible term alert) 'bingo wings' and generally firm up the upper arms.
Stand with your feet about hip width apart, knees slightly bent.
Raise your arms until they are horizontal from your shoulders, making sure your shoulders aren't hunched or tensed.
Rotate your arms from the shoulder until your palms are facing backwards.
Circle your arms up and back (small circles, not more than three or four inches) for a minute, making sure that your arms don't droop, and your shoulders don't tense up.
After, shake out your arms, then repeat, but with an up-and-down motion instead of the circles.
Two minutes a week really makes a difference. Four, even more so.
ETA: It's a lot more difficult than it sounds, I'm warning you.