-How do you prep your campaigns?
I kind of have two levels....the long term prep, and the short term prep. So pretty much Campaign Prep and Session prep.
I have a general idea of what the point of the campaign is, and where things are going, but this can change depending on PC actions and so on. So I keep it fluid. My campaign has a very elaborate backstory that involves a lot of D&D lore, and that's all set in stone, but what it all means for the campaign and what the PCs do is not. I want to keep it open so that they can do whatever they want.
Then, at the session level, I usually jot down a handful of bullet points. These are based on what happened in the last session, and where things are going, and what I think might happen this week. So I make this bullet list and then use that as a guideline. I prepare the likely villains that I'll need, and then use them as a guideline, too. I usually have a few villains that I can re-purpose....so if the PCs seem likely to be headed into a conflict with a thieves' guild, but then veer and decide to take on a mercenary company....I have a statblock that I can adjust on the fly from a guild member to a merc.
So I'd say the majority of my prep is for the long term stuff, as far as what's written down. In between sessions, I give a lot of thought to what I want to do the next session based on how things have gone, but I only jot down a few bullet points, never going beyond one page, and very seldom more than like 5 or 6 sentences in total.
-How much do you write before hand, and how much do you improvize?
I write a lot of notes for myself...the history of this organization, or the origin of this villain, and things like that. Anything that's in the past is what I want to kind of pin down to some extent so that I can accurately decide things on the fly in play. I find that if I know what an NPC has been through, and what his goals are, then I can very easily determine his reactions to the PCs and what they do.
I'd say that my campaign is pretty equal parts prep and improvisation.
-What is on your table during a session?
A large grid map that I can use to jot down info, or draw out situations, or that we can use for minis for complex or more tactically oriented encounters. I have a side table where I have a screen, the core books at hand, and a clipboard with my notes and a blank sheet for HP tracking and the like. We use index cards for initiative tracking, so I have those handy, too.
-And how long do your sessions generally last?
It varies a bit...we've recently moved from a biweekly game on Friday nights to a weekly game on Tuesday nights. The Friday sessions tended to be like 6 to 8 hour marathons, but Tuesdays are more like 4 hours.