You've got a pretty good workflow going, but you could probably be a bit more efficient. For starters, the more shortcuts you know, the better. Shift + Cmd + V will paste in place, versus the whole right click, move, click (but I guess that could be better if you're working on a mouse instead of tablet). Q will switch to the free-transform tool.
Also, the pen tool could be great for your shading work, since you seem to favor using straight lines with the occasional bend. It's basically a line tool that lets you click from point to point. If you want want to bend the line, click and drag when making the point. It's a somewhat strange tool that gets really easy once you're used to it.
It also could be easier to do shading lines before filling. That way you could onion skin, and you wouldn't need to be scrubbing back and forth (or follow Spazkid's method and shade from right to left, since onion skinning layers the frames counting upwards.)
Apologies if you already know this stuff and are just leaving it out of the tutorial for simplicity, or if you simply don't like these methods. I just figure any increase in speed is beneficial over the course of several thousand frames.