Coincidentally, I've been doing a ton of cape animations lately. Here are tome tips:
Mostly, I've been using two layers, and inside and outside layer, for the cape, and animating the cape points directly. It's not so difficult if you understand how to animate cloth motions. If you don't know how to animate flowing cloth, I would practice by animating simple waves before tackling this. It's not hard, just takes a little practice.
When drawing the cape, don't use more points in the shapes than necessary, and make use of the Magnet tool to animte it. The Magnet tools is much faster for this sort of thing than using the Transform Point tool.
Sometimes I need a version of the cape art that has more or less points in it. In this case, I'll create different versions of the cape in a switch layer. Also, I never animate the cape until all the body motion is done. It's a huge waste of time to animate the cape while your still animating the character's body. In fact, I will hide the cape group till I'm ready to animate it so I won't be tempted to mess with it.
If you want to re-use hand-animated cape motions, just copy/paste the keys in regular Actions. (Actions can now be exported/imported between scenes.) Some common motions I've saved are: Cape rising up and down (front view/back views), blowing to the side, whipping to side. TBH, I don't find much opportunity to re-use any motions since the character animations tend to be unique for every scene. If I ever re-use any of these motions, after inserting the keys, I'll probably have to take another pass with Magnet tool and retime the keys depending on the character animation.
When the character is standing still and I need the cape waving in a looped motion, I'll make this in a Smart Bone Dial. Just animate it once inside the SBD Action, set loop keys, and then set the dial's frame range to allow it to loop several times. Then, when you animate the dial, you can control the speed and timing of the wave motion. One time I needed to change the angle of the wave, so I added a smart mesh to warp it.
If you add these setups to the switch layer, you can switch between reusable loops and the hand animated motions. This is really useful when you have multiple looping setups in the switch. To smoothly transition between different loops, use a short hand-animated swithc layer/group to blend between them.
Another trick is the combine bones with hand-animated points. Just use one or two bones...this can be used to swing the shape in a nice arc. Then push the points around for the details.
It's also possible to rig a cape with bone chains and use Sketch Bones to animate it. Personally, I think the above methods will look better, and can be animated quickly once you get the hang of it. If you do go with Sketch bones, you might want to keep it to a single chain down the middle and also combine point animation to help it look more natural.
I've also used the FBF layer for cape motions. This is good way to knock out short transition animations to blend between capes using any of the above techniques.
Finally, make use of Onion Skin mode. I find this helps make it easier to animate capes (or any flowing element) smoothly. I suggest setting it to wires only--IMO, this makes it clearer to distinguish the current shape from the past/future shape than using the filled mode. BTW, Onion Skin even works between switched drawings!
Hope this helps. I actually have a bunch of other tips and tricks for cape animation. I'll see about making a tutorial on this subject. (Hopefully before I have to move on to something else and forget about it.)
