Looking for the best posture to sit

I have back-ache......the whole day i am moving here and there, tying to find the best posture to sit.

People claim that OL (office ladies) tend to have bigger hips...because they sit, sit and sit whole day in the office. In the end, their figures became pear shape.

I always complain that i have small hip compared to J.Lo's, but now i start to worry.
Seems like i am developing horizontally and at the lower part....*sob*

Can someone teach me how to sit??
I am trying this way now: sit straight and hold my breath. Use the abdomen instead of back bone to sit straight. I think that burns fat at the right place at least hehe.