I perfer to sit but in the enviroment that I DM in now, I have to stand most of the time. I wish we had a better gaming area but we don't. That still doesn't prevent me from enjoying the game.
oh wait sorry, i got that answer confused with another question...
anyway... when doing story elements and most NPC role-playing and such, i tend to sit down. when running combat with miniatures, i tend to stand, so that i can clearly see the combat (plus i get excited and nervous, making it hard to sit!)
When I sit, I sit in a higher chair than the other players. I sit during most NPC conversations and when the players are talking amonst themselves for strategy/planning/etc. I stand during combats because I like to pantomime the actions and reactions of NPCs while they are fighting.
I like having the freedom to alternate between sitting and standing. In general, I like to stand, but when my feet get tired, I'm glad there's a chair for me.
When I DM I get up and down all the time; all based upons my needs to be up and moving around or my needs to be at the desk, and whether or not I simply feel like sitting down or up and moving around.