The most important thing about a good posture is that it is a dynamic posture. It should always be moving, at least a little bit. A static posture is pain and injury waiting to happen. In fact static positioning is a common torture method for self inflicted pain.
Even though you are constantly moving there are some basic positions to follow to keep a good posture. Those positions support the natural S-curve of the spine. The spine, with its many vertebrae, is a flexible support column for you upper body that's kind of like s spring mount. It's able to absorb a lot of force and strain due to its design, but if it gets out of the S-curve you magnify the strain on your spine and can cause serious injury.
The spinal S-curve has a concave section at the lower back. The small of your back and lumbar should form a slight hollow. The spine continues upwards producing a convex area between the shoulder blades. So if you looked at a right profile of the spine (as opposed to the left profile) it would follow an "S" shape.
The S-curve and shoulder and pelvis alignment are supported with muscles balancing. The muscles on the left and right side should be equally strong. If you've had an injury or have a weak muscle in some other way then other muscles start to compensate. The muscle imbalance can throw the entire skeleton system out of whack and cause a lot of problems from back pain to Thoracic Outlet Syndrome.
Correcting any muscle imbalance is the first step to a good posture. Often this requires specific training of the afflicted muscle and should be done under the direction of a medical professional, physical therapist, personal trainer or someone else with training in strengthening specific muscles safely.
Ensuring your muscles are strong enough to hold your body in a good posture is the next thing to consider. This is most often a consideration for those who are overweight or aging. If you are overweight you are placing more stress on your muscles constantly and they will need to be strengthened to compensate. Or the excessive weight needs to be lost.
The elderly benefit from strength training to maintain muscle mass and density and that helps keep their posture in alignment. In general everyone will benefit from a balanced strength training regime in terms of supporting good posture.
The Standing Posture
Keeping a good posture while standing can simply be stated as "standing tall and proud." You should have your feet roughly shoulder width apart. Your knees should be in line with the middle of your feet and your hips should be about the same. Your knees should always be slightly bent. Locking your knees restricts blood flow and is a sure fire way to make yourself pass out.
Your torso should be balanced over your hips neither leaning forward nor backwards. Your shoulders should be back and your chest held out. This keeps your insides nice and open allowing the lungs to work without constriction and fluid to flow throughout the body without a problem. You head should be upright with your chin slightly elevated.
Arms should be held in a controlled manner at your side. Don't let them hang limply. Your muscles should always be in control and holding your body strong. If you place your hands on your waist you will end up raising your shoulders and that can be a bad thing if prolonged or done repeatedly. Instead place your hands inside pockets or fold place your hands into the small or your back. When you've achieved this posture remember to move from it. If you are supposed to be standing still make small deviations when able otherwise move a lot.
What you stand on can make a lot of difference in your posture. Concrete is one of the worst things to stand or even walk on. Hard, unforgiving surfaces can wreak havoc on your posture and body in general. Grass, carpet, and wood are better than stone, tile and concrete. When you do not have a softer surface to stand on be sure to use an anti-fatigue mat.
When you are sitting without any support for you back you should sit up straight with your torso balanced over your hips. The height of your seat should allow your upper legs to rest parallel to the ground or slope towards the floor at your knees. You never want your knees higher than your hips.
A comfortable range for your leg to torso angle is 90-105 degrees. This keeps everything open so blood and fluid flows freely. Placement of the feet should be whatever is comfortable and positions should change periodically. If your are leaning back slightly or have a short back rest then your feet should be flat on the floor in front of you. If you are leaning forward then you can tuck them in underneath the seat.
What you are doing while sitting has a major impact on your posture and an equal impact on what a good posture is for that task. Chairs or other seating supports should be adjusted to support these good postures.
Working at a computer or typing is best supported with a slightly reclined sitting posture with the back at about 105-120 degrees from the floor. A full back rest on the chair supporting your lumbar and shoulder blades is needed as well as a setup where your hands, elbows and shoulders are not raised excessively and your head is at a comfortable angle.
Reading and writing often requires sitting forward and working over a desk. In this case you want a chair that has a forward tilt of 5 to 15 degrees and a desk low enough to rest your forearms on without causing your shoulders to be raised. The chairs should also allow you to tuck your feet and lower legs under it without complications.
Playing a musical instrument while seated often requires a specific posture to support the motions needed for the instrument. Whatever the specifics are they should be observed whenever playing and practicing. Good posture in this case not only supports good health but is often necessary for proper playing.
What to Avoid
Keeping a good, dynamic posture can be difficult if you have developed some bad habits. Stay aware of your posture throughout the day and strive to avoid these common posture problems.
Do not hunch or roll your upper back. Keep those shoulders back, chin up and chest out. Also do not droop your shoulders. If you get tires or just lose focus you may top controlling you arms with your muscles and let them hang freely. Good posture needs muscles support so keep those arms held and those shoulders in place.
Do not cock your neck to the side. Unless you are still living in the eighties this is probably not a problem due to social pressure. Instead it often comes about with talking on the phone. If you tend to hold the phone between your head and shoulder you should stop. Utilize a speaker phone option if available. If you can not do that then you should invest in a headset for your phone. At the minimum you should attach shoulder support to your handset that limits the amount the neck has to bend to hold the phone to your shoulder.
Never lock your knees, or any of your joints for that matter. The joints should always be slightly flexed. Bone on bone, full extension should be avoided. It can damage the joint and reduce fluid flow to the extremities. If you lock the knees while standing it will cause you to eventually pass out. Usually in about 5-15 minutes.
You should also avoid cocking your hips to the side while standing. A sachet of the hips while walking is not a problem and is often quite normal body mechanics. And a side to side movement or shifting of your weight over your hips can be helpful but cocking the hip well over to the side is not.
Cocking the hips to one side while standing is similar to locking your knees. It can constrict fluid flow around and through the pelvis. It also shifts your balance point over to the side causing more strain than necessary on your body. At the same time it pulls the spine out of a healthy S-curve. And lastly it tends to let the bone rest on bone on the joint instead of having the body held by muscle which can causes irritation and bone damage.
Try to keep from raising your shoulders upwards. You probably raise your shoulders hundreds of times a day without even realizing it. Every time you rest your elbows or forearms on an armrest that is slightly too high for you it pushes your shoulders up. If you place your hands on your hips it pushes your shoulders up. If you hold hands with someone taller than you it pushes your shoulders up. If you carry a purse or shoulder bag you often shrug your shoulder upwards to keep it on.
Raising your shoulders, especially just one of them, causes a significant imbalanced loading across your upper body. One raised, or dropped, shoulder can throw your posture severely out of whack.
Keep your feet healthy and springy. Allow them to keep the natural splaying (where the feet turn out slightly with the toes wider apart than the heels). Feet are not meant to be perfectly straight. They are also meant to be dynamic with motion in the ankle, metatarsals and toes in every step. Clodhopper, hard soled shoes should be avoided.
Good posture is paramount for good ergonomic. It should be dynamic and strong. It is also affected by your individual body type. So use these guidelines and find what works for you. A good posture should keep you free from pain, allow you to stay flexible and provide the strength and motion necessary to perform your task without undue stress on any component of your body.