I first make sure I know what tastes my players are into. Lots of action? Lots of dialogue? Dungeon crawling? City intrigue? Stuff like that.
Then I write broad adventure hooks based on that. Also based on what I think would be fun for them. For instance: we did a dungeon crawl last adventure, next adventure is about them lost in a series of haunted canyons. I try to vary the "flavors" of each adventure. I label each adventure as such so I know what "flavors" I'm using too much or which "flavors" I haven't even given yet.
After that, I come up with what the antagonist is all about. Now, keep in mind, the antagonist isn't always living beings. Such as the adventure to escape the haunted canyons. In that case, the antagonist is a location. Anyways, if its a mad wizard trying to break into a library for instance, I pretend to be the mad wizard and make my plans.
-I make up stats for NPCs.
-I make up personalities at same time.
-I make up spare NPCs, just in case PCs wander off and do 'extra' stuff.
-I draw up rough maps.
-After that: I 'wing it' all.
Some of my DMing style also includes trying to keep some air of realism in motivations and action/reaction. If a PC doesn't take the side quest that helps save the farmer's daughter from the wizard, I don't just leave it at that. Like the daughter just ceases to exist afterwards. I make a note as to what happens to her because she isn't rescued (as well as her Father's reaction)....and that may come up fairly soon....or in the future (in a cool "ah, remember that one time when...." type connection).
Or also, if they choose not to stop the wizard from raiding that library, I sigh and break out another back-up adventure. But.......I make a note that this wizard now has gotten that special book that makes him that much more dangerous now. This can come around and effect the PCs in different ways.
Also, something I do a lot to add realism: If the wizard plans entering the library 4 days from the start of the adventure, I don't 'push back' the time just because the players stopped to goof off on the way. The wizard still goes ahead as planned regardless of what the players do. Now, this may sound tryanical....but it actually makes for cool gameply IMHO. The adventure suddenly turns from stopping the wizard....to now chasing after him and getting the book back. Because the theft has already happened. These unexpected changes may frustrate other DMs, but I like 'em! It keeps my job from being predictable!
I always like to narrate in such a style that makes the PCs feel like they're not being railroaded. I'm not sure how other groups are (I'm sure it differs from one group to the next) but with my group, the less hints and pushes I hurl their way to get on track, the better they stay on track. I guess because, after they get the initial story starter, they feel the rest is up to them. If I keep making it obvious they need to go a certain direction, the more they either resist it or dislike it because they can start to obviously see what the story is all about. They like it when they can figure it out for themselves.
Yes, this can make for PCs going WAY OFF the story sometimes, but really...I haven't encountered that too often. Plus, if you know the motivations and personalities of major key players around the kingdom you're playing in...its easy to free-form and wing a new adventure up on the spot.
Kind of a simple way to describe my basic adventure writing.