Stuff like:
-Background Music
-Posture/Seating
-Favorite controller
-Self-hyping
Anything
In games like Darks Souls, or fighting games where theres no point to listen to the ingame music I like to have my stereo on playing some background sounds, generally some rock music (Seether, In this Moment, Evanescence, Devilskin to name a few) or some rap (mostly just Tech N9ne, Eminem or Hopsin/Funk Volume: I seem to prefer lyrics to a good beat while gaming cause I can focus on the words and sort of 'zone in' to the game.)
During the first playthrough of a game however its all ingame music, have to watch every movie/cutscene and I will even generally let the entire credits roll on the off chance there is something missable right at the end.
Seating: Nothing too special here, I often slouch or lean with my leg hanging over the arm of the couch. Only thing I really can't handle is playing laying down, just throws me off.