Ergonomic Chairs and Accessories are a good start when dealing with low back pain, sciatica, or neck pain. Unfortunately, one can still slouch in an Ergonomic chair. Slouching places your spine at a disadvantage and will often exacerbate your pain.
This is not the fault of the chair. It is due to your posture.
I often encourage the use of a digital timer placed accross your office and set for 15 minutes. Take note of your posture at the beginning of the 15 minutes, and at the end of the 15 minutes. You will find that your posture has started to fade by the end of the 15 minutes. If you can have the patience to continue this for a few weeks, you will begin to realize when your posture fades and self correct.