How can I cut my petrol costs?

Every week a Guardian Money reader submits a question, and it's up to you to help him or her out – a selection of the best answers will appear in Saturday's paper. This week's question I'm now spending around £100 a week on petrol, driving from Oxford to north London for work. I'm interested in car sharing, but does this system really work? And what are the other ways I can cut petrol costs? Is it merely a matter of taking my foot off the pedal? What are your thoughts?