Well I sit with the room's door behind me, so I prefer it to be closed especially if there is anyone home. I really dislike having someone standing behind me while I'm sitting, so i usually pause the game if i can, and face the "intruder" until she or he leaves. (Besides, I consider playing or using the computer an activity for one person, so no need for two in the room...)
I also like getting another char and resting my legs on it...although this makes some games somewhat harder.
Oh and eating chocolate is nice too!

Well, generally, light are going off as soon as I am launching the game (except for small light in my back, only monitor light hurts my eyes). The next step is getting doors closed, turning on speakers, securing rations such as water, scotch, chips, chocolate.
Aside from going to bathroom, I can stay like this for hours :)
I have a crappy computer chair, so I can't have the luxury of experiencing new angles, as I could in fact break the chair :)

I cuss and make growling noises when I get into frustrating positions/levels during a game. That's about it.

Headphones when she's home or I am playing something where I need to use my mic, otherwise the volume is up on the speakers - bass really brings out the explosions in games. Sugary beverage to hand (soda, or a Hurricane or Zombie most likely). Blinds closed to block the sunlight. When it 's hot a deskfan is blowing in my face to cool off, unless I'm using my mic, then it's time to turn on the AC.

If I'm lucky and the kids are in bed and the spouse isn't watching TV... blessed silence except for the game...and that's on low volume. Maybe a stout on weekend nights hanging out on an RPG with my buddies, otherwise Diet Coke.
If not, I'm trying to play while having two girls under 10 watching over my shoulder or else having to stop every so often to Be The Parent. :( Obviously the gore factor has to be lower if they're around!

Destroy their minds now, why wait until later? :)

I'll sometimes play while watching T.V. or watching a movie I've seen several times. Usually with a rum and coke and a few chips to snack on.

I've started to play some games with my Wireless 360 Controller, using the Pinnacle Game Profiler to map all the buttons. I'm a traitor, I know. :)

I usually play at my computer desk (even when playing my DS, as I can rest my arms on the cushioned wrist rest for the keyboard), usually with a desk lamp, or with the lights off completely for games that seem to require it (zombie titles, Bioshock, etc.). I usually have a glass of water sitting nearby for longer or multiplayer sessions. I always use headphones to play for better directional location (only have 2.1 speakers for the computer). Oh, and I suppose the main quirk is that I always use a PistolMouse for all FPS/TPS games - there's just something about the feeling of pulling an actual trigger over just clicking a mouse that adds considerably to the immersion.

While gaming on my couch, I almost never have my feet on the ground. For some reason I always have them on the couch... Just realised this a few seconds ago haha, guess that's what makes me comfortable!

For PC, comfy ass chair, Bose quiet comfort headphones, lights off, blinds closed, 24 inch monitor on, 2 or 3 beers ready to go and I'm off!
For Consoles, comfy ass sofa, 5.1 pioneer system on, 42-inch LG plasma on, blinds down, controller in hand, headset on, and I'm good to go!
Cheers!

I'm usually sitting on my bed as my computer chair for my desktop is taken up by my laptop/gaming machine. and with a sega genesis PS2 and N64 all connected to my 12 year old 13" tv i switch a lot, i have cookies on hand usually and my whiskey somewhere on my floor if i feel the need for it, usually i don't though. Guitar hero must be played standing though as it's hard to be a rocker whilst sitting on a bed, in the musical way anyways. other than that i don't drink a lot (alchohol or otherwise) so i never have anything near besides my whiskey. which is about two months old and is 375 ml and still probably has about 275 ml left.

YEEEEEEEEEEEEEEEEEAAAAAAAAAAAAAAAAAAAAAAAAAAAAHHHHHHHHHHHHHHHH!

For consoles, I usually have my PS3, Xbox 360, Wii, Dreamcast, N64, NES, SNES, and PS2 plugged into a power strip. I then have an S-Video cable connected to my 720p projector (Read: Not TV), and can swap it at will by plugging the other ends into the older consoles.
For the PS3/360/Dreamcast, I run through a VGA cable, though some Dreamcast titles don't work with VGA.