Well...what helps me to keep my arms warm (and my back, chest, etc so I'm not sure if it will work for you) is a Columbia Omni-Heat shirt. It uses your own body heat to keep you warm by reflecting the heat back at you. I bought the shirt a size up from what I normally wear so it is not too tight. I don't know if this would work for you because it would keep your whole upper body warm and not JUST the arm...but figured I might as well mention it. I've thought about getting the arm warmers myself but have just never gotten around to it.

Also...if a particular part of my arm is hurting me I also buy those stick on heating patches (the generic versions of thermacare). They help me a lot with flares.