I stand, my nerves won't allow me to sit. During trading I stand and once I have a position I get on my tread mill and run till I exit. I ran 5 miles once during the dull mid-day once, on a bright side Ive lost some weight. Kind of like Forest Gump I guess.
A very comfortable office style swivel chair with arms and excellent lumbar support, height and tilt adjusted to be correct for me, and it also leans back for when the markets are quiet and I can relax a bit.
Good seating is very important when you are going to spend so much time sitting in it...