It's all about the Plot Arc.
I use several plot arcs of various shapes and sizes. A plot arc can be something that pops up every once in a while but not too involving like the PCs slowly discovering an evil cult's involvement in local affairs and their eventual halting of the cult. It can take over the focus of the game, like travel to a certain place with a particular goal in mind. But, the thing about them is that they have a beginning and ending point.
I usually have several plot arcs going on at the same time, and hooks into plot arcs integrated before the arc actually needs to start. I always have at least one plot arc or ready plot arc for each PC and usually two arcs occuring for the entire party. Right now my arcs include:
1) Bruin obtained a poweful sword, but found out it was stolen from his clan long ago and has returned it. In order to become worthy of the blade he must prove himself.
2) Brackran is in search of an uncle who can teach him about who he is and why he has these strange magial powers. He recently found out his uncle is in Sigil.
3) Vadan's teacher has ties to a local adventuring academy, but has dissapeared. She wants to find out the mysteries surrounding him.
4) Cien needs to find a giant snake to return to a wizard who needs it as an ingredient for some unknown purpose.
5) The party has been hired to get a sword commissioned in a far away city. They have convinced the smith to make the weapon and now must return it safely back to the one who hired them.
6) The wizard who hired Cien has been acting strangely. He hasn't been sleeping and is researching binding circles. Why does he need this snake? He is a good friend of the PCs and they owe him much, but something is definately wrong.
And lots more. The trick is to have an arc ready in reserve to add a bit before or a bit after one of these is completed by the PCs. Also, what looks like a simple plot arc can turn into a major plot arc depending on PC and NPC actions (as in #1) or a major plot arc can appear to be a simple plot arc at first (as in #6).
So the PCs have definable goals, and these goals can turn into major aspects of the campaign. The PCs can have a sense of accomplishment and really have a sense that they are accomplishing things, because many things they do have a definate sense of completion to them (because they are completed). Also, large story arcs can have smaller arcs within them. Always a good thing.
This also allows me to put a time frame on things. I can say, within 2 sessions #1 should be completed. Within 3 sessions #4 should be completed. Within 4 sessions #5 should be completed. Therefore, I should introduce be introducing some new plot arcs soon. Knowing this, I can seed the beginnings of something new happening within that time frame so that when the PCs are ready to tackle it, it won't just jump up at them, and they can have time to prepare if need be. I want to introduce a new villian, for example, so I can make one of the subbordinates in the next 2 sessions have a reason to want revenge against the PCs, he will then become a major villian in his own right, assuming he lives. Then, while in Sigil dealing with #2, I can spring him on them.
I think it helps the world seem more "organic."
