There are three comfortable positions for playing my bass while sitting down, as shown in the photos below.
(click on each photo to
I use this position when playing intently or learning difficult passages. The bass is propped across both legs, raising the instrument to a higher and more visible position. My legs fit nicely into the two smaller arcs of the lower body. Notice that the inverse horn of the upper body curls comfortably around my chest, rather than poking into the breastbone as is the tendency of a traditional body style.
This is my favorite position when playing with a pick. I put the bass on my left leg at roughly a 45 degree angle, and put the arc of the heel of the bass against my right thigh.