Bad Posture Is Bad for Health

Inflexibility
Reduced range of motion in the joints
weight gain

These all contribute to poor posture.
The Importance of Good Posture
Good posture allows the optimum distribution of force throughout the body so the musculoskeletal system can function properly and protect against injury and deterioration.
If the spine is not held straight, the internal organs are crowded into less space, which can reduce their effectiveness.
Poor posture can cause health problems, for example, sitting in a bad position at an office computer could cause stress headaches, tensed shoulders and lower back pain.
Practical Tips to Improve your Posture
At home

Spend 10 minutes each morning doing some simple stretches. This is particularly important if you have a sedentary job and tend to go from bed to sitting in your car to sitting at your desk all day.
Take a few moments each day to focus on your posture; mentally work your way down your body, straightening up your spine, bringing your shoulders down away from your ears and pulling in your stomach muscles.
Avoid sofas and low-seated chairs. They may be the comfortable option, but as you sink down your knees become higher than your spine. This puts pressure on the hips and causes the spine to flex.
Spend a few minutes laid out flat on the floor, with no cushions or support. Do this at least once a day.

 
At work

Try to avoid relying on the back of your chair for support. Come forward in your seat, keeping a small curve in your lower back. This will help to prevent the pain caused when you hunch over your computer.
Don't cross your legs when sitting because it over-stretches one side of your legs, slowly misaligning the pelvis and spine.
Carrying a large heavy bag on one shoulder puts unequal pressure on the spine, so opt for a small bag and regularly swap the shoulder you carry it on. Alternatively, use a backpack that sits on both shoulders.

Bad posture can also be corrected by exercises which strengthen and restore flexibility to the body's core muscles (abdominal and pelvic floor muscles), which may have become weak after years of poor posture.
It is often found that greater awareness of your posture and consciously changing bad posture habits can bring noticeable improvements after a few weeks.
                                                              Source: Healthy Muslim