I was told by an instructor once that arms in the latin dances (for the lady) should never be straight but always bent at angles. I found that when I combined that with allowing my body to move my arms (as was previously suggested here) it worked pretty well. Put on a good song and practice just moving, like free dancing, in front of a mirror.
A while back they had a decent cuban motion lesson on ballroomdeancers.com, but I don't know if there's an archive of those. It was animated and had decent detail for the mechanics; I had already learned many of the same things from my own instructor, but I did think that it was easy to understand.