If your main technique is plucking, the part where the neck enters the bass’ body should be on your belly button, give or take a few inches. Low enough to allow the forearm of the plucking hand to not lean on, but against the body, high enough to allow the fretting hand to reach all frets comfortably without moving the instrument. Players whose main technique is slapping or thumbing might benefit from strapping the bass higher. In their case, the neck may be kept in an almost horizontal position. Normally however, the bass neck should hang in a 30-45 degree angle.
The wider the angle, the more room to move for both arms/hands there will be. Experiment with both height and angle to find a comfortable position.
There are two drawbacks to a steep neck angle:
For a steeper neck angle, you may mount a second lower strap button 1 – 1,5 inches higher up. Using it may tilt the bass' body backward a little, which is also good for resting the right arm against (not on) the bass' body. However, the fingers of the fretting hand should find no difficulty reaching all strings without flexing the wrist.
In the video, the lower strap button is not in the middle, but higher up. This is shown right in the beginning where the strap button is pointed to the camera.
A second angle is important also, though most instructors neglect it. The bass' body is best positioned not parallel to the pelvis line - the line between the tops of the hipbones - but on the plucking hand side of the abdomen, resting it on both the abdomen and the right side hipbone. This brings on many advantages:
A possible drawback is fatigue in the left arm, because it is kept in a more horizontal position. The best angle of the bass with the pelvis line depends on the player's lenght. The longer the player, the further away from the left shouder the headstock should be. However, the elbow should not be extended too far as is explained here. When playing near the nut, the angle between upper arm and forearm should better not be over 135 degrees.
Special problems with positioning the bass are experienced by obese players. They will have trouble to keep their instrument steady. For healthy plucking, the bass' body will have to be kept in the normal position. The lower rim should not be lifted by the abdomen. It should not be strapped above the abdomen. It is best kept on the plucking hand side of the belly as explained above, but only if this position allows you to reach the lower postions easily. Unless your obesity is rather severe, this should be the case.
Keeping the bass on the fretting hand side at normal height will force the left elbow behind the pelvis line, at the risk of shoulder problems. If you strap higher and rest the lower rim of the body on the abdomen, the fingerboard may be tilted backward too far, forcing the left wrist too flex too much. You will run the risk of carpal tunnel syndrome. If strapping high goes together with a neck angle of 45 degrees or more, and thus moving the headstock to somewhere near the ear, the left elbow will be flexed constantly, risking cubiltal tunnel syndrome.
The fretting hand should never have to support the neck.
Strap your bass, stand up and spread your arms. The bass should stay where it is, hanging perfectly balanced. If the neck dives, corrections are necessary. Using an elastic strap may work. Adding weight to the bass’ body is less advisable. Instead, relocate one or both of the strap buttons. With most basses, neck dive problems may be cured by either moving up the lower strap button or by extending the upper strap button.
Neck diving is risky not just for the fretting hand, but may also contribute to problems with the plucking or picking hand. Players who both pluck and press the thumb down on a pickup to keep the bass steady might develop wrist tendonitis or various elbow problems; if they also flex their wrist to the inside, carpal tunnel syndrome is a serious risk. Players who pick might be tempted to press the bass against their body, putting strain on the forearm and making it impossible for the wrist to pass on the force of the picking movement to the elbow, such at the risk of wrist tendonitis.
As many beautiful types of wood are very heavy, some players are experiencing problems with their strap shoulder or with their neck. A heavyweight instrument will draw the shoulder to the front, while the shoulders should be kept wide and relaxed instead, in accordance with correct posture. A broad, comfortable and somewhat elastic strap will divide the pressure and absorb the minor but nasty shocks the shoulder muscles have to take from moving around with your bass strapped. Other players prefer a padded strap. Start trying different straps as soon as your shoulder starts to hurt. The mere change of strap may result in putting the pressure on a different spot.
Although some types of wood may sound better than others, most of the sound qualities come from the bass’ electronics and a quality amp, so there is no real need to torture yourself with a heavyweight bass. If a different strap does not provide relief, buy a lightweight instrument. Or again, the ideas from the video might be helpful. A waist strap will carry quite some of the weight. As pointed out above, you also have to deal with the shocks on your shoulder muscles when moving. To minimise those, an adjustable waist strap is very helpful. There are two drawbacks. The waist strap will determine the strapping height of the bass. After you strap your waist strap, the shoulder strap can only be tightened, not adjusted for height. Secondly, if the third strap button is installed on the lower horn of the body, like in the video, the bass will be more to the left. Try to reach a compromise. Experiment by loosening the waist strap bit by bit and moving the bass down and to the right as far as it will go, still allowing the waist strap to carry most of the weight of the bass.
In case of very serious shoulder problems, sitting down or using a walkup stand may become necessary.
Nice bass, madam!