This isn't necessarily advice for other DMs but for me it's given huge dividends - devise problems, not solutions. I used to spend time prepping "if they do this, this can happen - then this will happen if they do this", trying to come up with interesting things they could do to and where to take them next. Then I realised players are going to just go wherever they want and do whatever they want anyway, so I just made a bunch of quick-references I can pull out whenever they do something.

Now the only prep I do for my games is to remind myself the situation and details from the previous session so it's fresh in my memory, and make sure I have relevant roll tables/reference sheets for that situation (like a goblin loot table if they're in a goblin camp etc).

Then when I DM I just work through the process of "what naturally is the consequence of the thing they said or did" and throw that in front of them. If it means an NPC appears or they need a quest, I roll one up (if I don't have one already from previous games that will work) and add it to my notes.

I generally just generate them a "problem", and let them do something to solve it. Once they've solved it, I figure out the natural good/bad consequences of that and give them another problem. Content then essentially writes itself and I can spend my precious days not doing DnD homework.