I sit and stand in rather equal amounts during the game. It's vital for the battlemat that we use on the table, and it let's me feel like I'm in charge, towering over everyone. It also let's me feel a bit less restrained, and I can get all crazy with my descriptions and stuff.
I always use a DMscreen...I just can't picture gaming without one...(I love having a place to hide my die rolls, hide my face if I want to, keep notes secret, and tape up reminders for myself) but this makes standing even more necessary for a lot of the game.
I'm a performer, so standing is fairly common for me. *shrugs*