The most important preparations I make for my D&D games happen after the session. Making dungeons and monsters is fun and valuable work, but it’s important for the game to respond to the PC’s actions. Right after a session is when the events of play are fresh in your mind. It’s the best time to think about what consequences might arise in the future.
I think it might be a useful tool (for me, if not for anybody else) to put together a post-session questionnaire. Something to prompt the referee to examine the elements of the session most likely to bear interesting fruit in the future.
Who did the characters wrong during the session?
Do the wronged parties know the characters are responsible? If they don’t know, will they care enough to find out? How will they try to get back at the party? Will they try to hunt them down and take violent revenge? Will they attempt to pressure the party legally or socially? Will they concoct elaborate schemes, or take a more direct approach?
Don’t limit yourself to NPCs who’ve appeared in the game. If someone died, did they have a family who will want vengeance? If the characters won something, does that mean there’s someone else who lost it? Good deeds earn the ire of villains as easily as evil deeds earn the ire of goodly folk.
Did the characters do anything stealthily during the session?
Did they go anywhere they weren’t supposed to be? Did they leave behind any clues that might make someone want to find out what happened, or to improve their security? Just because nobody heard the characters prying open a door doesn’t mean the damaged door won’t be discovered later.
Is there anyone, aside from employers, who benefited from the character’s actions?
Do they regard the party as selfless and heroic, or do they take a more cynical view? Just how far does the party’s good reputation spread? Will they admire the party? Seek to aide them in the future? What is the most interesting way they could show their appreciation? Can they provide the party with information, services, material, or future jobs? Will the party inspire them to become adventurers in their own right? If so, will they attempt to join the party as hirelings, or will they pop up in some future adventure to aid the party?
If anyone died in the course of the party’s adventure, what sort of hole did they leave behind in their community? What effect did they have on their environment which will be lost now that they’re dead?
Did people depend on the person who was killed? Will they lose their jobs, or become vulnerable to attack, or have their legal system disrupted? Will someone rise to fill the vacuum they left, or will there be a period of chaos followed by a significant change in status quo?
The same questions might apply to any objects the party stole.
What information did the party spread around?
Did they make any new discoveries, or reveal any secrets? Will the people who know this new information keep it to themselves, or will it quickly become public knowledge? How can the information be exploited for personal gain and profit?
Did the characters do anything which the authorities, or the public at large, will have feelings about?
Did the characters engage in any public violence in a normally peaceful space? Did they flout local custom in some particularly egregious way? Did they rile up a mob? Did they make anyone afraid? Did they make shows of wealth in front of the poor, or shows of poverty in front of the rich?
Are the characters known to be the cause? If so, will there be legal consequences? If not, do people want to know who is responsible? In either case, will there be some change to the local laws or attitudes?
Did the players express any desires or interests you might use to engage / hook them in the future?
Did anyone mention a goal that they have? Perhaps an item or territory they wanted; a power or social status they wished to achieve? Did they particularly like an NPC who could reappear as a friendly face / be injured to make the players angry? Did they particularly hate an NPC who could develop into a bigger antagonist just to annoy them?
A truncated version of this is going right in my POST_SESSION.txt