You've been DMing for a while now, let's suppose.
Let's set the stage.
One of the biggest pitfalls for novice game masters of any tabletop RPG is knowing how and when to say no to players.
Here's a little secret -- I don't do a lot of formal planning for running my D&D games.
There are a lot of excellent Dungeons & Dragons setting books out there that will give you all the details you need to run a game in their setting, and those settings (also known as campaign settings, world settings, campaign worlds and/or simply campaigns) are often excellent.
Sooner or later, someone always suggests this one.
Every Dungeons and Dragons Dungeon Master has been there.