Here are some useful sources of info on Sciatica:
Written Explanation of what a lumbar herniated disc is here
At the end of this clip is a good model of how the herniation occurs and extensions may help. See from 7 minutes here

Good explanation of what causes sciatica: here and then to "Sciatica Self Care" with Dr Best here he is a Chiropractor and goes on to have what looks to be a good program for $29.95 here see below video for details as video just shows what you get for the US$29.95.

Another good you tube video about what causes sciatica

McKenzie Method
These videos from Spine Health Phsiotherpy in Armidale Victoria demonstrate different variations of the McKenzie Method Exercises here.
Here is a You Tube video of the McKenzie exercises for herniated disc an sciatic pain.
This video talks about the load on the spine in different positions.
This Chiropractic Article discusses Williams Flexion Exercises (WFE) (essentially flexing) and Mckenzie Method of Mechanical Diagnosis and Therapy (MDT).
Here is a Chiropractic site with McKenzie Exercises for Sciatica
Here is a protocol that I decided to follow

This Meta-Analyis of the McKenzie method vs passive treatment showed non significant benefits for lower back pain (LBP).
Here is another study of research which also indicated slight benefits of McKenzie Therapy over other modalities

Exercises to avoid
here they are:
Leg Press
Straight Leg Exercises
Twisting Exercises