The bass should be held in a fashion that permits it to resonate as freely as possible while allowing access to left and right hand needs. The right hand must be able to bow without hitting any bouts or corners, adjust the endpin accordingly. The left hand must be able to access the entire fingerboard without "adverse" (or excessive...unnatural...) wrist bending. Two solutions are effective, one is similar to cello technique where the weight of the left hand (and right with the bow) are used to facilitate dexterity, this is most effective when seated as this involves adding more weight to the strings of the bass....support is required.
The other is interesting and very effective. Stand upright (not like a primate!) in a solid stance, feet shoulder width apart and with the weight of your body seated between the tripod like fulcrum of each foot (the foot tripod; the ball, the heal and the pinky toe side), be aware of where your weight is and make it comfortable (many people don't even know how to stand in a comfortable position, regardless of what they're doing!). The bass will contact your body at two points (besides the strings...) the back edge of the upper bout rests on your hip bone...this is the fulcrum. The back edge of the lower bout rests on your left knee (move the bass into this position, not your body, you should be standing comfortably upright). The fingerboard is very "up and down", perpindicular to the ground. This creates a slight "roll" forward that is stopped (or controlled) by your left knee (the hip being the fulcrum). This essentially uses the weight of the instrument to depress strings with the left hand and "dig in" with the right as the instrument sits in the left hand and under the fingers with virtually no effort; the thumb is free, the wrist is beautifully natural throughout all registers, the instrument has all of its major resonating surfaces (the top, back and sides...the sides actually do make a very big difference) and you are moving fluidly and in balance with the instrument. This technique is to thank for countless (painless) 6 hour gigs back to back on a regular basis! It has been effective for the classical etudes I work on at home and the various genres I perform on the bandstand on a regular basis. I believe this to be the proper "upright" technique; the Rabbath...are adapted (and highly effective) cello techniques. Do what makes you sound good and keeps you out of pain!