We all know that backstory enriches PC roleplaying as does campaign background enrich play overall. However, notice that both words have something in common: the prefix BACK. Both these elements are behind the scenes forces that should rarely rise to the fore. Certainly they bubble up or peak through at times, but ultimately they are setting related. They should not take the role of primacy in any game. Endless discourses on background, history, personality and motivation can become quick game killers.
Take for example when you walk into a new room or building to meet someone or for a party or some sort of conference. Seldom are we totally absorbed in the design, decor, colors, structure, history, and architecture of the place. You do notice it and it lends important feel and tone to whatever you are doing. But instead of being totally absorbed in the setting you are engaged with what might be happening in the room; who is there that you might interact with; what activity you might join in or generally what you will be doing in the room. Especially pertinent would be any assessment of dangers or things that might challenge, threaten or thrill you. The focus is on the action in which you are or will be involved. Less so on the setting, which while important, only represents a background with which you will interact peripherally at best. (A possible exception to this is an environment that might represent the whole challenge, such as climbing a snow covered mountain.)
So no matter how lovingly and painstakingly you may have designed your world or scenario, don't bore your players with long irrelevant descriptions. There are numerous reasons for this, but first and foremost it rarely has immediacy or immanence. What we mean by this is that it isn't important right now. Players need things to engage right there, right in front of them. Playing of active roleplaying games requires that players get to do something. Don't let the play drag through boring or irrelevant descriptions of the surroundings, campaign history, royal lineages or the like. No matter how engaging they may be to tell or read or were for you to write. They are flavor and texture that come up now and again, usually in the form of tantalizing hints. For instance, the PCs are about to head off into the woods beyond the edge of the town and are asking a local villager for directions. The grizzled old man explains the way and mentions they should be careful crossing Black Foot bridge, "It's cursed doncha know." The relevance to the story at hand may be minimal, but it's relevance to the villager is high. He lost his child there once to a mysterious drowning accident. And the rumor that the bridge was cursed rises from ancient lore that plague ridden villagers were driven out of the village over the bridge into the woods. The oozing black wounds of their feet leaving behind blood stained, shadowy footprints on the bridge's stones. Hence the name black foot bridge. The feeling villages then pronounced a curse upon the village from the far bank of the river. And ever since strange drownings and disappearances have occurred there.
What this little tidbit does is expand the game beyond it's distant boundaries. It is immediate and immanent. The only way out of the village to the woods is across the river, and the only easily passable trail leads across a cursed bridge. What may happen there when they cross? Which cross they must if they wish to be able to achieve the woods and their adventure beyond. This immediacy of importance, the immanent nature of the threat moves the story along and enriches it, deepens it. This level of engagement is desirable if done in proper measure and does not become a distraction from what the party is trying to achieve.
Focus on encounters of immediate relevance to the PCs. Either because it presents an immediate danger or must be faced presently as a challenge or point of interest. Such immanence catches PCs attention and focuses them on the story at hand, at what is happening right now. They should, therefore engage with the scenario and your story will unfold. The trap right in front of the PCs is much more important than the sorcerer at the bottom of the dungeon. Don't cut the trap short. Give it its due. This is the time for that trap to shine. Someone could very well die right here. Make it memorable in its immanence of danger and urgency. If you want the sorcerer to be important then you had better weave him into the PCs lives' right _now_. Or at the least seek to make the PCs so interested and confronted with getting to the bottom (where the sorcerer is) that they want to go there.
Storytelling should focus on action not plotline. As GM you should know how the campaign structure and history will impact the PCs as they move along. Truly engaging events come up because they are important to the PCs right now. That is immediacy. Your job as GM is to weave important encounters into the scenario. A simple dungeon populated with miscellaneous monsters that exist solely to fight along the way to the end of the dungeon does not for a good story make. Not that it can't be a foundation for a good story however. The way that happens is to make each encounter unique and present enough to continue the engagement. Otherwise a story is in danger of fading into forgetfulness.
For example let's take the example of the dungeon above. Level 1 is inhabited by goblins, level 2 with hobgoblins, level 3 with more unique randomly determined creatures and level 4 with the evil sorcerer and his personal minions. Instead of having the PCs just plow through room after room of goblin fodder make the threat more immanent. The goblins are afraid of their sorcerous boss, not to mention their hob overlords. so they seek to actually do their job of keeping PCs from getting to the bottom levels. But they realize that the PCs outclass them in all but numbers. So our goblins choose to set up some ambushes and some blockades once they find out the PCs are exploring their demesne. They seek to lead the PCs on a merry chase through the trapped regions of the 1st level, wearing them out and exhausting their meager supplies. And now we come to the trap mentioned previously. It's significance is much greater to the PCs for several reasons now. They just chased a band of fleeing gobs down this corridor to be confronted by the trap. Up ahead the flickering remains of their last torch reveals a T-intersection. They think the little buggers went left. But if they stop and listen they can hear many more goblin whispers and scuttling apparently coming from both passages. They have already been ambushed several times. The goblins set fire to the exit passage and the PCs know that attempts at escape just means a suffocating death if they retreat. The threats are very dire and immediate. Moreover these gobs are hiding something for sure. Just what the PCs don't know, but they certainly want to find out. They've spilt too much blood already not to. That is immediacy. The threat is now, and it must be faced. They will never forget this adventure, and they aren't even a third of the way through yet.
So, good GM storytelling isn't just about developing a cool scenario or setting everything up ahead of time. It is about presenting immediate and immanent threats to the PCs in the context of the land, area or situation in which they find themselves. Developing and setting up this tension requires a mind that is itself present in the scenario. You must be in the action as much or moreso than the PCs. What would scare them, challenge them, make them struggle right now? Shifting the development of the adventure at a moment's notice is fine if it keeps the players engaged and their PCs challenged. There are of course dangers with this element. The idea is not to run the PCs ragged or to kill them with constant unending challenges and trials. There must be a careful balance that is best achieved by being in tune with your players and the current state of their PCs. I have always done this by feel, but numerous helpful record keeping tools are available for the GM who prefers these aides. Whatever the case, keep your players guessing, keep them engaged and keep the action high.