In all(?) the current published adventures, the way they suggest to get the player's involved is to leverage their obligation in some way or to do a hand wave and say, "this person you know contacts you about a job."
I find this to be unsatisfying, especially the further into the game you go. I would much rather have adventure opportunities come from NPCs that the PCs have actually met over the course of the game.
What tips do you have for introducing and maintaining these NPCs and what successful examples do you have from games you've run?
Just to head off something that I'm sure will derail the conversation, I'm not looking for advice on creating memorable or interesting NPCs; there's plenty of advice online for that. What I'm looking for is what ways you've found to introduce the NPCs and develop the relationships with the PCs such that it makes sense to the players that this person would trust them with a job.