Left foot on the floor, right foot on the chair. Left elbow on the desk using my left hand to prop up my head. Right arm leaning on my right thigh and my right hand either on the mouse or arrow keys. Plus copious amounts of slouching.
Sometimes i have my right leg on the floor and my left leg on it, Sometimes i sit on the chair with both legs crossed together... I can be very random when it comes to sitting depending on the situation.
I think I throw myself into the random category. I have to change positions every few mins or so because the position I was just in gets uncomfortable. Writing this, I am just sitting normal with both feet on the floor, about change positions to sitting on one of my feet.
I use a speaker as a footrest, lean back on the back of my chair, both arms on the armrest while the left is supporting my head, and my right on the mouse. I find seats without footrests very uncomfortable. I absolutely need a footrest.