An excellent question, one that I'm forced to grapple with constantly as someone who teaches survey courses.
The main thing I do is pick out a couple of major themes I want to cover for the term, and then try and select examples that will function as a narrative for these themes. For instance, in early American History, I often try to emphasize the struggle between State and federal power in the early 1800s, and explore the differences between the idealism of the so-called "Founding Fathers" with the political demands of the time--as it happens, the Louisiana Purchase is a great example of both of these themes and illustrates the conflicts with the Jefferson administration.
It's much tougher for World Civ--thus my themes have to be chosen carefully. For Ancient/Medieval World Civ I focus on the conditions necessary to create a "civilization," the factors that go into the "fall" of a civilization, and the formation of some of the great world religious and philosophical systems as a lens for seeing social change. For Early Modern Civ, I basically encourage students to reflect on a question: "How and why did Europe--a cultural backwater in 1400--manage to conquer the rest of the globe by 1900?" For Modern Civ, I try to explore the various facets of what we mean by "globalization."
Different instructors tend to structure their courses in slightly different ways, but there are usually some events that are so important that they fall into the narratives regardless.
In the future will schools be forced to offer a cliff notes version of everything history, since the source material will be so large?
This is all survey courses ever are anyway: Cliff's Notes. The best you can hope for is to expose students to some of the major trends, how historical research is done, and how to analyze historical events. If they're really interested, then you point them to areas of specialization. But I always tell students, even if they take a year's worth of my survey course, they've only really scratched the surface.