1. NPCs are the Supporting Cast
It is all too easy for a DM to fall into a habit of using narration and long expositions to cover background and color for his setting, and this is in the face of an old writing axiom: Show, don't tell. Instead of sermonizing about the rich cultural heritage about the peoples of Punkia, let the NPCs do the talking. Describe their clothing, the food they are eating, and the like. Make the people part of the setting. If I mention NYC you can already imagine the throngs of men and women walking up and down the street in their business suits, while cabbies jostle for position and urbanites and hipsters prowl the secondary roads. These NPCs don't need lines, they don't need anything other than a description. They can talk and pass on snippets and rumors to the PCs, or let you vent some of that creative energy you've expended in birthing your world.
2. NPCs are a Social Immune System
A common theme in many vampire and modern magic games is the idea of minding your own business and keeping your nose clean because if you don't the mass of humanity is going to come crashing down on you. In other games, PCs can get used to putting their thing down, causing as much trouble as they want and getting away with it. If the PCs start running amok, a subtle way to rein them back in is to use social interaction, the NPCs become less friendly towards the PCs, and instead of getting discounts for smooth talking the business owners they find themselves paying full or higher price (wary shop owner versus genial shop owner) or get a reputation for trying the schmooze and cheat their way out of bills. City Guardsmen have families, and after the PCs go on an urban rampage, even if they are exonerated later the locals will still remember that these guys are guard killers aka cop killers. If this sort of action doesn't work, or pushes the PCs to intimidation and criminal acts, feel free to bring in the larger guns such as bounty hunters, knights dispatched by the king, or representatives from the Guild of Adventurers upon Return.
3. NPCs can be Turks
What is a Turk you ask? Turks are can-do men. Rather than creating a large amount of background material for an incidental character, the GM can use a Turk. If you take the decades of hype, cool gadgets, and dozens of spin off stories, Boba Fett is a Turk, his skills are Tracking, Exotic Weapon, and Piloting. A Turk is basically a professional, with 3 to 5 improved skills and enough of an advantage that he can get the drop on the NPCs if needed, or could be a magic item maker to employ the PCs, or an elite hireling that can stand shoulder to shoulder with the fighters to protect the softer PCs. These NPCs can easily grow into fully fledged NPCs with ideas and plots of their own, but the main role of the Turk is that of a tool for the GM.
4. NPCs can be Butterflies
It is easy for NPCs to start looking like the PCs, walking blocks of combat stats and dump stats. Butterflies are pretty NPCs, the sort that the PCs should interact with. These are not princesses or queens, those are full fledged NPCs in their own right. Butterflies are defined by two characteristics, one they have no physical combat ability and will flee. The second is that they are social/appearance/charisma based and work very well within the social paradigm. The Butterfly is the stunning whore that the other NPCs and PCs fight over who gets her first in the night, or the bard who has no combat skill but is a skillful player of the lute and seducer of fair maidens. Butterflies are generally to be protected.
5. NPCs are People Too
It is fairly common in my experience that GMs like to have mooks and redshirts fight to the bloody last against the NPCs just to prove to how badass the boss they work for is. NPCs are people so should act like people. NPCs should have morale, and act accordingly. If the PCs butcher two thirds of their number in 3 combat rounds they should turn tail and run. They can lay low while the PCs take on their boss, or fake an injury. Humans have an amazing ability to make up excuses as to why they can't do something. But on the contrary, humans can also display amazing fanaticism and zealotry. It all depends on the situation. Criminals are rarely fight them to the bitter bloody end, and religious types aren't going to cut and run at the first morale check. A merchant is going to know how to haggle, and shouldn't be blindsided by the glib tongued PC, and rogue NPCs are going to know when to hold em, fold em, and most certainly when to run.
6. Borrow People You Know
Everyday we are surrounded by people we know that our gaming group doesn't. These are coworkers, people we encounter with regularity such as the staff at our favorite. It is easy to just borrow names or mannerisms, but a better perspective is to borrow their points of view. This is especially valuable if you know people who have radically different perspectives than you do. You can project them into your NPCs to make them more than just extensions of yourself. For example: I have coworkers who are: devout Christians, conspiracy theorists, welfare recipients and upper income. They all have VERY different views on life and can make for some interesting NPCs when they are used. The Christian merchant won't offer a discount for the PCs because he tithes and if he doesnt make a profit he takes a loss after the tithe, where the conspiracy theorist merchant has all sorts of crazy rumors and doesn't trust the PCs at all. These are just a few examples.
7. Back to Basics
NPCs shouldn't carry the PCs, they should have a mannerism or peculiarity that makes them stand out just a bit. This can be as little as always having a pipe, to the fashionable cut of their vest, or always being hungry when they meet the PCs. A random habit or observation can quickly grow into a full fledged NPC. The Cigarette Smoking man from X-Files was supposed to be in one or two episodes, and the Joker was a one time villain for Batman. Don't be afraid the roll the dice and let a new quirk grow into a character. And don't try giving them all funny voices, that gets irritating.