Are you talking about introducing new PC's, NPC's or both?
So what I try to do is link characters together through events and story. As the events gain momentum, they sweep up more and more PCs into the action. The PCs won the day (as intended), but had to flee the city (as intended) and are now bound together. There's a lot more there (motivations, visions, and quests) driving the PCs that I didn't go into, but I just wanted to illustrated how I approach getting characters together.
Yeah, it's certainly more work, but it is exactly the work I enjoy as a DM. Cranking out encounters is okay, I guess, but I really love developing story and setting and watching it all work together. All of my campaigns feel like a story more than they feel like what I'd consider a typical D&D model of moving from dungeon to dungeon.The main disadvantage is that you really need to know your players' PCs before you start, so that you can come up with PC appropriate hooks into the adventure. That means work for the DM and cooperation from (occasionally lazy) players.