Honestly my approach to GMing comes down to the concept of knowing your setting. Your world is not always going to be canon game world, but if you know how your world works then the game tends to flow well.
By this I mean:
1) Know the core NPCs. The characters contacts are important, have a discussion with your players about who their contacts are. What is the relationship like. How do they know each other.
2) Know the core locations. Know the inhabitants of the core locations, what the areas look like, what the areas feel like.
3) Know who the movers and shakers are. Something I typically do is have a short list of some of the city's top shadow talent. Typically it is the best team or two, outlined in broad strokes, names and jobs, and the top few individual runners. Beyond that the top end fixers, arms dealers and talismongers are names that are floated around. These are the folks that the players generally want to do business with, but have to establish the credibility or connections to get the introductions.
4) Create a few urban legends for your game. These really tend to add to flavor and setting. For example, my games are all set in the Toronto sprawl(home sweet home), and everyone knows a guy who knows a guy that was at the CN Tower when some crazy street samurai tried to pull a gun at a meet gone bad, and simply disappeared. No one will draw weapons in the Tower as a result of the urban legend. It is the ultimate spot for a safe meet.
Anyway hope that helps some.