The Dicky Bag is definitely worth the money, particularly if you want to stop for a picnic or pub lunch. Nothing else we have tried has prevented the slight whiff!
When we went on our first holiday with our dog to Yorkshire we noticed a distinct lack of any kind of bins in carparks so we often had to drive off with a bag or two and it was grim. The Dicky Bag is now a must have, we keep an emergency one in the car. As a PP said though it is all too easy to forget about it and it is pretty grim after a few days.
For shorter walks a holder to clip on the lead keeps it away from hands.