Sitting: Your body does not move much when you use the computer, so always make sure sitting comfortably in your chair.

Chair Position: The computer keyboard should be about as high as your stomach. It should not be placed at your chest or shoulder height, as you will have to reach up to use it.Your arms, shoulder and neck could hurt if the keyboard is too high for you.

Computer Slump: You need to always learn back in your chair and rest on the backrest when you are typing. Do not slump forward, as this could  affect your back.

Keyboard Position: While using the computer always keep the keyboard close in front of you. Do not put it so for back that you will need to reach forward in order to use it.

Typing Lightly: You do not need to press hard on the keys. A light touch on the keys will work just fine.

Using a Mouse: Always use a light touch on the mouse while holding it as well as clicking.Too much force on the mouse will tire your hands and arms.
Monitor Position: The computer monitor should be right in front of you, A little lower lower than your eyes. If you have to look the side, up, or down your head and neck could start to hurt. On the other hand, If the monitor is too low it could cause you to slump in your chair.

Blinding light: Always make the light coming from the screen is the same amount as that around the computer monitor. Move the computer so that the windows and other lights are not directly infront of you, or put a shade over them to avoid this problem.
Break Away: Take a break about every 30 minutes. Get away from the computer and do something else. Also, pay special attention to your eyes. If they hurt you, Stop for a bit. Look away from the monitor every once in a while and focus on something for away, for a few seconds. This will give your eyes some much needed exercise.

Taking care of your body while at the computer means you will feel better, and play better.

In day to day life it is not possible to obey all the above rules. But I suggest to obey the above mentioned rule as much as possible because our body is a beautiful creation of God and we have to take care of it.