I've got a few of those doggie car harnesses. They work well unless you've got a dog like HoudiniBailey
, who wriggles out of the harness and than plops down next to it.
I've ended up using them most when we dogsit for a colleague who has a dog who gets out of his collar and runnnnnnnns when he's frightened by a noise when we're on a walk. The harness seems to keep him feeling reasonably secure (and is harder to get out of than the collar alone).