What are the checkpoints for good posture while sitting?

When sitting, your feet should be facing forward with your knees and hips at 90 degree angles. Your spine should be straight with shoulders relaxed. Finally, your head should be neutral with your ears in line with your shoulders.

Sitting with no back support increases the force on the spine about 40 percent more than standing. Leaning forward when you sit causes even higher forces. A reclined posture with your chair back at a slight angle often works best to keep you pain free. When you sit in the chair, your buttocks should press against the back of the chair. Your feet should be flat on the floor. Your knees should be slightly higher than your hips, and you shouldn't have to strain to see your computer. Use a footstool, if possible, to ease pressure off the back.

Make sure your chair has an armrest, as it helps to take some of the strain off your neck and shoulders. The arms of your chair should be able to go under the desk to help prevent the position of sitting and leaning forward. Using an armrest should make you less likely to slouch forward in your chair. 

Maintaining a neutral spine while sitting is very important (as it is while standing, running, etc...)! When you are sitting you should keep the following in mind (from the head down, so easier to remember)

1. Keep you head positioned over your neck- not drooping down or with your chin jutting out.
2. Try to pull your shoulders back- avoid the rounded, hunched over posture!
3. Sit up tall- think about lifting your rib cage off of your pelvis- creating a gap between the bottom of your rib cage and the top of your pelvis. 
4. Keep your pelvis in the "neutral" position- think of your pelvis like a bucket of water. You want to keep the water in the bucket! You don't want it to spill out the front or out of the back of the bucket.
5. Try to keep your knees bent at 90 degrees and your feet hip width apart and straight ahead.