Just wanted to say how much I appreciate the GM's advice on how to run the DFRPG - the information in the back of vol 1 - and not just in there, but sprinkled throughout the book - rarely is specific to the Dresden Files, but is easily applied to other games as well. Spirit of the Century is much the same way - I got SotC because of the DFRPG and its a tome that's full of wise and sage advice as well.
Probably the biggest thing I've learned from it is how to interpret failure so that its not an outright fail, but a complication, or it adds a twist, or it changes the consequence & fallout. Its these things that push a story along, keep your players engaged and makes a scene compelling.
What are your go-to GM resources to help you hone your craft?