I believe I've mentioned before, but I change the way I prep everything from sessions to NPCs fairly regularly. It's like how I'm doing things stops working, or I forget some of the key steps that I am supposed to focus on which makes it lose the 'magic' so to say. I am always looking for new/better ways too, so it's not long before I find something and then I go off in that direction.
That said, while I've gone through a bunch of ways to make NPCs I don't know I've ever had one stand out to me as a solid, quick way to make an NPC that covers all the bases I need. Obviously there is more needed for a major NPC than a minor one, but sometimes an NPC is straddling the line where you're not sure if they'll be a big deal or not until you're in session and need that.
At present I am using a method where I try to define their weaknesses, wants, needs, allies, and enemies. It works ok, but leaves out some qualities that could be useful. I've previously done 3 aspects like I was making a FATE character which can also work, and you can scale to 5 aspects for bigger characters.
How about you? What method do you use? What have you found works well for you?