Whatever is needed... to clarify, I usually sit but if there are many players and I need to be more evident, or if it's a complex fight and I need to view the combat mat clearly, I'll stand.
Though, when waiting on responses, sometimes I'll get up, walk around, rehearse what NPCs are going to say, or just play while waiting for the PCs to pose their responses.
I imagine that if I ever got down to doing it RL, then I would be standing whenever I'm describing something or needing energy. But looking up stuff, rolling it, bla bla, that's sittin' work.
With (usually) six players at the games, plus the combat board, munchies, miniatures and all their books, it gets a bit crowded at the gaming table. So, when DMing, I usually have all my stuff on the kitchen counter (I have a combined kitchen/dining area, so it's very convenient). Gives me lots of room to work, and puts all my books and notes out of sight of the PCs without the need for a screen.
As for sitting vs standing ... mostly I stand and walk around, since I am often sketching stuff on the map or moving minis. But I do keep a chair at the counter, for when I get tired
Generaly I like to stand (and walk arround) when the attention is necesary "on the table" as when the players have to work stuff out amongst themselves. I'll also stand/walk when roleplaying NPC's and the situation requires it. (eg a Justicar interogating the party should hover ominously over the wee players...)
When polay gets more technical (as mostly happens when we're in combat) I prefer to sit behind my screen with my notes and reference material close at hand. I'll soon stand up again though
as a Dm's screen is perfect to hide notes and dice rolls behind, but often obscures the table which is especialy annoying when we're using a battlemat and mini's
I start out the game sitting, but as the game progresses (and the caffeine), I wind up standing, then pacing, then walking around the table. When the miniatures come out, I have to get up and look at things and move even if I don't want to.