Every year dogs die after being left in the car. Even a 'few minutes while I just ...' can cause your pet real distress when the weather's hot.
There are loads of walks and cafes/pubs on the DwD site (not just the motorway dog walks) to help you plan long journeys with dog stops where you don't have to leave your dog in the car.
There's also useful information about what to do if you think your dog is suffering from heat-stroke on http://www.keepyourdogcool.co.uk/ - with posters to print off and put up in your local supermarket/library etc
Dogs Today magazine have also launched a 2011 campaign 'Don't Cook your Dog' and you can get free stickers from their website, just click on the link below.