With all of the bad 419/Nigerian emails sitting in your inbox, I can expect you to be skeptical when I tell you that I found something good on the net from Nigeria.
How to maintain a good posture is a fine article by Adeolu Babatunde. It is a quite comprehensive look at the issues of posture as it related to lying down. Next week, posture in sitting will be discussed.
I have found it helpful to do the stretching exercises my physical therapist recommends. Do some core strengthening exercises, such as the ones highlighted at The Mayo Clinic. By keeping your core strong, your back won't put itself in a position to cause that sciatica pain you are having.