I mean, naturally, 'other than write the adventure and make maps'.
If we have a quick pickup game, I don't do all of this, but for any game scheduled in advance - I do!
Once the players tell me which characters they are taking on the adventure I go through my notes on each character to see if there are any hints, hooks, etc. I need to drop. Will Ember recover another lost memory? Will Brigid's sword tell her some lost bit of lore? Will Thorin's bracelet give him more orders?
Then I go over my campaign notes and jot down a few things they will have heard, such as: the king has a new nephew; the price of coal is up, so the caravans to Riverstone need more guards; there are rumors of war drums from the Stone Hills; Llewellyn the Black won the singing competition in Timberlake.
I also determine if they need something more. Should the old lady that sells apples utter a prophecy? Magical Weather or a comet to portend great peril? I don't do it often, but I make sure that at least every 4-5 sessions to point out via an encounter that the world of the PCs is strange and full of magic.
Then I go through my NPC box to see if there is any need to mention interaction with an NPC not directly part of the adventure. This can range from 'you got a letter from Marla talking about the farm and how they have new kittens and thank you for saving them from the hobgoblin' to 'an envoy from King Snurri meets you and gives you a gold armband on the anniversary of your defense of his kingdom' to 'a halfling named Deighton Tomlolly shows up at your door; he's the son of Durnan Tomlolly, a torch bearer that died on your first adventure. He says that you promised him a job when he was old enough to travel to the city'.
I figure out the most likely route(s) they will take too and from the adventure, refer to my campaign books and jot down the names of the inns and taverns within the villages along the route. I double check the NPC box for any more possible NPC interactions.
Then I pre-roll random encounters for the route(s) and generate any needed stats
And then I approximate the travel time to, in, and from the adventure and generate weather for that long plus 2 days.
I consolidate this all into a page or two of notes arranged chronologically that I can use at the table.
This usually takes about an hour.
There are things I have on hand at the table to help, too. I typically keep;
10 of each PC class with name and full stats, gear, and personality
10 of each NPC class with name, full stats, gear, and personality
2 small randomly-generated bands of mercenaries with gear
2 of each sort of expert hireling with name and basic personality
20 names each for both sexes of each PC race
20 Tavern names
20 Inn names
Names of 5 merchant ships and 5 warships
20 village names
I mark them off and use them as needed, updating maps and notes after the session.
-My NPC box
-The weather generation charts
-Custom encounter charts
-a set of 'pocket encounters' - like the old Side Treks in Dungeon magazine, I have a handful of lairs with monsters for low, medium, and high level parties. Some are friendly, too.
And... the players still surprise me almost every session.
How do you prep?