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 high for you. 
Adjust your chair so your feet (and shoes) are flat on the floor/footrest and pressure is spread evenly across your rear and upper legs.

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.