Once you master these, there's a sense of "Now what?" Well, what I did, just for me, was to put a crossover at the peak of the curve.
Now if you put a crossover at the point where you change edge, that's a cross stroke. We're not having none of that here.
This is what I do.
1. Timing and placing the crossover in just the right place means I have to change the edge in just the right place, and be solid on the edge in time for the crossover. If the serpentine isn't very big, this is a challenge.
2. In order to do the change edge after the crossover, I have to do a decent underpush with the inside (outside edge) foot when the free foot crosses over. Then get on the inside edge and be solid before doing the next edge change.
This cute little drill takes a few minutes to do, and helps me with quickness, edges, and upper body control.