...and other complex backgrounds...
I came across an insanely detailed background drawing from Oliver and Company on Animation Backgrounds (a great resource, btw).
At first, I tried drawing it freehand, the same way I would a portrait. That proved difficult, but insightful.
So I decided to draw it in a way I'd never drawn a background before, and I found it extremely valuable to use for complex cityscapes and aerial shots.
I also find writing down my process helps me remember it better later, even if I never look at what I wrote down ever again.
So here it is:
1) Thumbnail/rough it out
2) Draw the ground plane or surface on which the buildings are built on
3) Grid the surface/ground plane
4) Draw boxes for groups of buildings (not for individual buildings yet)
5) Small boxes for groups of small buildings (usually closer to the shore/edge of city)
6) Medium boxes for groups of medium size buildings (usually further into the city and makes up most of the cityscape)
7) Large boxes for large individual buildings and landmarks (usually peppered in between medium size buildings and closer to the city centers, also usually closer together than spread out evenly)
8) Leave empty spaces for parks, streets, and other undeveloped spaces. It also makes the drawing clear to understand
9) Break the the boxes down into small buildings. Space the buildings unevenly so as to achieve a naturalistic looking skyline.
10) Use reference to capture as many landmarks as possible to give the drawing flavor and a sense of time and location.