I mentioned that I am playing under the jurisdiction of an awesomely capable GM in a fantastic Pathfinder game. Prior to this, he also led us through a 3.5ed campaign set in my homebrew world. I have years of playing experience with this guy, and I learned some things about my own GMing that could stand vast improvement. I'll share them one at a time. Here's the first:
Continue each character's backstory.
If you're like me, you have trouble coming up with an engaging plot line. But well-imagined characters already give you seeds for creativity. Well into our last campaign, I realized that every story arc placed a different party member at the center of attention. This is a superb approach, because it calls upon each player to involve himself personally, and not drift toward either party dominance or passivity, two extremes that can weaken a team.
In our particular campaign, each PC's experiences worked synergistically at the climax of the long-term story to directly affect the story's outcome. Once you've tied a couple of side plots together, things begin to write themselves.
I'm resolving to avoid writer's block by asking myself, "Why is this character adventuring? What should happen to develop this character?" Whether you respond directly to a character's needs (like offering a coveted item in a quest), or arouse that character's frustration by opposing them (for example, by killing off an NPC villain that the character wanted to exact revenge upon), you're appealing to that PC's player on a direct, emotional level. Systematically work through the group this way, and you'll never be stuck for a story idea.