1) Adjust your chair height. Most office chairs have multiple movable back pieces and can go up and down. Make sure that you adjust the height of the chair first so that your elbows are at a 90 degree angle to the desk. If your feet can no long rest on the ground comfortably, then get a box to put under your desk that you can rest your feet on. Tucking your legs under your seat and crossing your feet causes you to hyper-extend your big toe and can lead to damage of the first metacarpal-phalangeal joint.
2) Adjust the back of your chair. Most chairs allow you to move the back piece up and down. Move this piece until the small of your back is being supported by the arch of the chair (see the video for a visual explanation).
3) Move your arm rests. Some chairs will allow you to move the arm rests on your chair. If this is possible with your chair then move them so that your arms can rest gently on the rests. If they are too low than your shoulders will automatically shrug to keep your arms even, creating unneccesary stress on the shoulders and neck. If they are too high then they will push your shoulders up, creating an uncomfortable position that again will lead to unnecessary stress on your shoulders and neck.
4) Take away the clips that raise your keyboard. You may have never thought to look at the bottom of your keyboard for these risers, but almost every keyboard has them. If you do know that they exist then you probably thought that having those risers on the back of the keyboard were put there to help you, but they actually make it so that your wrists are in a constant extended position. This along with the repetition of hitting keys all day is what eventually causing carpal tunnel syndrome. Your hands should float freely on your keys with little to no bend in the wrist.
5) Move your screen. This tip is one of the most crucial for neck pain and headaches. You want to make it so that the screen is an arm's length away from your eyes. Then move the screen up or down so that the top of the screen is at your eye level. This will allow you to use your eyes to look down at the screen and help to stretch out the muscle in the back of the neck. If the computer screen is too high we have a tendancy to move our necks forward and up (like a turtle). If they are too low we slouch down to see the screen.
6) Get up and move around. This is probably the best piece of advice I can give you. No one should be sitting at a computer desk for 8 hours a day with only one lunch break. Every ten minutes take a mini break, 30-45 second and stretch, then every hour get up and walk around for a minute or two. Most large companies will give morning and afternoon breaks for ten or fifteen minutes, do not spend this time playing games on your computer. Get up, walk and stretch. Movement is key to living a healthy, pain free, life.