Why I can't adopt a good posture

I don't refer to Kamasutra's.

I know that a good posture is the key to avoid chronic lesions. As programmer, I have to take special care when using the computer, so many hours sitting down, typing, could end with my health.

I'm conscious how a good posture can reduce the fatigue that being sitting down that much can make. But I can't avoid it, sometimes I sit to the front, to the back, leaned, to one side, just I miss faced down, but I will get sick.

It has been this way since school. The only thing that was over the back of the chair was my head. Even then I was aware of these things, so, when I realizes, I sit again well. It last no longer before I returned to the starting position. I can shun it.

I try to have an eye on me to avoid this bad habit, but it's very difficult. It seems that I keep doing this, mistreating my body and not getting up from the chair.