First Secret of Prep: It will always, always, always add value to your game and make for a better session IF (and this is a very important if) you focus your prep on the stuff you can’t improvise at the table.
Second Secret of Prep: What you can improvise effectively will depend on your own strengths as a GM, it will change over time, and it will vary based on the system you’re running. I talked about one facet of this in The Hierarchy of Reference, but it applies across the board. Maybe you struggle with having dynamic battles featuring clever tactics, so you spend a little effort prepping Tucker’s Kobolds. Maybe you find it easier to run Pathfinder monsters if you make a point of highlighting feats you’re unfamiliar with and jotting down a note about what they do. Personally, I know that I get too tight-lipped with NPCs revealing the deep secrets of a campaign (because I ruined a campaign once by getting too loose-lipped with those secrets and it’s a Pandora’s Box you can’t close — if the PCs don’t know something they can always learn it later; if they learn too much they can’t forget it), so personally I focus a certain amount of effort on prepping exactly what NPCs know.
Third Secret of Prep: Some stuff you find hard to improvise can be made easy to improvise if you prep the right tools. Procedural content generators are an obvious example of this. But it can also include stuff like “if you’re bad at coming up with names on the fly, prep a list of names”.
Particularly valuable prep targets, of course, are the things that can never be improvised on the fly. Props and handouts are perhaps the most obvious example of this.
Short and sweet but very helpful.
I’m a new reader, but I love this blog, it’s already really helped my DMing (I managed to turn Serpent’s Skull part 2, a linear set of railroaded encounters, into a pseudo-hexcrawl. This has literally saved a dying campaign. Thanks for that, by the way!)
One of my greatest weaknesses is a lack of organization, so posts like this are always appreciated.