Ask your friends and neighbours if they want to share their cars. If you and your neighbour shop at the same out of town supermarket it makes sense that you go at the same time and share the car. Swap around every week to make sure everyone gets the benefit of leaving the car at home. Ask around at work and see if anyone shares the same journey as you, and if they do sort out a rota and share the driving. Every car left at home is a saved journey and they soon mount up.
If you're really serious there are websites for car sharing (e.g. www.nationalcarshare.co.uk), which will put you in touch with people making journeys or needing a lift - many of which have been set up by county councils or businesses wanting their employees to think about their travel needs. Not only will you potentially save lots of money, but you might meet some new people and are helping save the environment too.