1. Connections: In general players (and I as DM) love when there is a callback or other connection to previously experienced stuff in what is going on now (and that includes stuff from character background). Is the estranged husband the party druid wrote about in their background the recurring villain? Is the young woman you rescued from the necromancer's familiar coming to the party wizard to be an apprentice? Does the sage that hired you for your first ever adventure writing you a letter many months and levels later to ask for help again? Is the one rogue dinosaur egg you did not retrieve in an early adventure the source for the bugbear chieftain's dino pet later on? etc. . .
2. Not Everything is Connected: Resist the temptation to make everything connected. Let there be encounters, adventures, NPCs the PCs run across that have nothing to do with anything else, to allow a source of fresh stuff in the game and to avoid the dissatisfaction of too much narrative convenience or convolutions of way too complicated plot that can feel like you are just constantly adding more.
3. Follow Through On Consequences: I think it is the DM's job to warn/inform players about things their characters would know or notice (or had known and the players forgot) because while the PC is in the world, the player is not, esp. if you are like my group and only get to play every 3 to 5 weeks and it can be hard to keep up with every single detail. It is no fun to have some bad stuff happen because you were not aware of something the character should have been aware of and would not have forgotten. And yet, once that hurdle is cleared in any given instance and the PCs make choices, whatever makes sense to happen (esp. with the DM's greater knowledge of the various moving pieces), should happen.