You might try repositioning your bass or changing your sitting posture so as to relieve pressure on the major veins and arteries. Blocking them is what causes the numbess.
For example, when I read the paper on the can

, I rest my elbows just behind my knees, and my legs are
severely asleep by the time I get up - however, when I play sitting down, I rest my bass as close to my belly as possible, and no problems. In fact, I often (bad habit, I know) sort of curl around the bass, and rest my chin on the upper horn - I like to feel the vibrations through my jaw. (Chicks also say it's adorable, like I'm cuddling it.)
Also - what are you sitting on? A couch, hard chair, bar stool, computer chair, etc can all be conducive to bad posture while playing. I like to use a drum throne - well built, heavily padded, springy, adjustable height. I set it so that my thighs are angling slighly towards the floor, and I tap with my right foot.
And finally, what sort of bass are you playing? A modern style skinny body will concentrate the force into a smaller area, essentially letting the weight penetrate further into your leg and closing off veings deeper inside. Try a bass with a wider body, like a P, and see if that helps.