Sit comfortably!

Image via WikipediaLower back support is very important. Sit up straight and with a nice posture: Back straight, neck straight, shoulders up and back, eyes forward, etc.
Office chairs can be quite nice as far keeping a good posture and you should use them to your advantage. My chair, however is not good on the back. I find that if you don't have a lot of support, you can use one of those padded notebook thingys. (A "pad-folio"?)
They're rigid enough that they keep the back straight and the padding is just enough so that your spine is protected. The other benefit they offer is to remind you to not lean back too hard and sit up straight because you probably don't want to crush your nice black pleather pad-folio.

The article below on "Standing versus Sitting" is very interesting too.

Related articles

• Ergonomic Work Chair: Know Before You Buy
• Office Chair Back Support - How Useful is it for You?
• Importance of an Office Chair to Your Back
• Standing versus Sitting