Welcome! Sounds like you are in good hands with Cassandra's school.
For me, I like "natural" arm movement - which can be really unnatural to achieve. So for most of the time I use arms and hands to frame and draw the eye. The trick is then not to make them stiff and rigid; a small movement of the wrist or a softening of the shoulders will do it. I learnt most one year when I was pretty ill (recovering from brain surgery) but had signed up for an eight-day intensitive with Aida Nour. I didn't have the stamina to dance for the whole time but I could spend many hours following her arm, hand and shoulder positioning. If you cannot actually follow someone with good arms dancing for a few hours (doing choreography is likely to be less useful), I'm sure you could get similiar benefit from following on video/DVD
The other side of arms is to occassionally do an accent (I think of snake arms in this category). For it to be effective you need to be able to hit the exact angle, position and texture on the first hit - pretty much the opposite of the skills above. For this, working with a mirror is great. Pick a move you want to accent. Play around until your arms are something like you want. Now. Eyes closed. Do the move, (eyes open) and straight into the arm accent. Check the look (or use a video camera). And again. And again. And ...