1. How do you sit?

Sit up straight on your chair with your back against the backrest. Which of these best describes you?

Your chair is set too low for you. 
Sit with your feet (shoes) flat on the floor. Raise the chair until you can just still rest your feet flat on the floor. Ensure that your lower and upper legs are at a 90 degree angle, and that your weight is evenly spread over your rear.

Sitting in this way spreads pressure as evenly as possible across your rear.

more information

See picture on the left for the best setting.
Having full contact with the chair when seated, and fully supported feet, allows pressure to be spread as evenly as possible.