twistedcat has a nice technique where the leash is half hitched about the abdomen. It stays loose till the dog pulls then the rear legs are effectively lifted off the ground. As soon as the dog stops pulling, the half hitch falls loose. The technique worked well on my brother's beagle and he's a big puller.

It also stops the dog from pulling out of the collar, although your lovely mutt appears to have a big head so that may not be an issue.

-Tass