I have got no more else in my head other than push ups to work out my upper arms. But a friend told me that I should also couple it with something else. Otherwise, there would be an imbalance in the development in my upper arm muscles and in form and shape. I don't want them to look funny with having incompatible shapes. I asked her to recommend me and she said go out and find.
I suggest you try swimming, every kick and every arm stroke becomes a resistance exercise -- and it's well known that resistance exercises are the best way to build muscle tone and strength. In addition to toning visible muscles like pectorals, triceps and quads, swimming also helps improve the most important muscle in our bodies: the heart.
Practicing a fight is a great way to slim down into your arms. Practicing Muay Thai for example is great for that. You work your arms a lot, mainly with jabs in the air, so it doesn't create so much muscle, it mainly tones and defines the muscles in your arms.