Each DM preps in different ways. Here's how I do it.
Prep my campaign
I've got the gift of being pretty creative, and I work part-time, so I write (or type) most of my games. That being said, nobody can correct me if I ran a section "wrong". As long as the players have fun, as well as I, I like to think I'm doing my duty as a DM. I still will read a few things on occasion, or tweak things around based on party size.
I write quite a bit, but improvise many NPC's & Villains
I set a good chunk of time aside to write my homebrew games. If I write FAST, I may write 2-3 times a week. If I write slow, I may write only a couple hours once per week or less. That being said, sometimes you cannot predict where your players go. So I have to use what I have in mind, improvise it in game, and write it down on a campaign tracker later. It sucks when you forget what you improvise but your players remember, but writing it down later insures that you won't forget as often.
What's on my table
1) Custom DM screen with world map, encounter table, & rules I need to reference
2) Laptop - Stores NPC's, Villains, and some monster stats.
3) Hot coffee/espresso or a cold beer (session dependant)
4) Dice rolling container with: Dice, pawns, and room to roll.
5) Folder with important items current play area.
6) Monster manual & DM guide standing up on the floor.
Session Length
Usually only 2-3.5 hours. But we tend to have sessions weekly or bi-weekly. That said, I have to be a quick DM and I draw my maps beforehand!