I've got the great privilege to be developing the Tyrant's Grasp Adventure Path here at Paizo. Although I know quite well where the entire 6-part arc is going, I'm currently just over halfway through the development. I've done enough now to categorize in my mind the connections I'm working hard to make across all the adventures. They make good points for any adventure author to keep in mind, whether writing a simple 5-room dungeon or an entire campaign. I call them seeds, callbacks, and themes. All of these stand outside the actual plot of the adventure, but serve to supplement it.

Seeds are encounters, NPCs, or locations that are only hinted at in an early spot, but come to full fruition later on. An example is meeting a masked NPC who gives a bit of information before disappearing in an early adventure, finding out who she is what she wants in a later adventure, and finding out the extent of her true motivations in an even later adventure. Good seeds keep the players guessing and engaged in moving forward.

Callbacks are the reverse of seeds. A callback is a minor tag to something that the players already saw in full in an earlier spot. This works particularly well as an acknowledgement of an earlier task, "Hey, you're the people that saved that noble," "I'm a guard from that watchpost you saved," or "That bejeweled scabbard you found in the ogre lair was my family heirloom, and I'll pay you double its value just to have it back." But even a passing reference makes the players feel like their earlier efforts had an impact.

Themes are commonalities throughout an adventure or adventure path: that the key villains always have figurines of wondrous power, that the PCs keep meeting greedy demons, or they end up having a lot of fights while traveling on rivers. Done poorly, these seem like lazy repetition to avoid work--the way backgrounds in cartoons keep looping to save on animation costs. Done well, they show the PCs connections between groups or scenes and tie the entire adventure arc together. Done exceptionally well, they show the PCs these connections subtly, contributing to an overall atmosphere.

I can't take credit for the groundwork in Tyrant's Grasp, however; the adventure path was outlined by the inimitable Crystal Frasier. I'm glad to be walking in her footsteps on this road!