You could try the "nothing happens until you stop that!" method.
As soon as the dog has the leash in it's mouth you stop what you are doing, stand still and in an upright position with no eye contact until the dog releases the leash. This may take a while at first and you may think you are getting nowhere fast but if you keep it up the dog will get the message. This behaviour is usually the dog showing excitment and anticipation - leash means we are going somewhere. If you stop going somewhere when this behaviour occurs then it becomes counter productive.
When the dog releases the leash a quiet, calm, "Good dog" is enough (you don't want to raise excitement levels again) and carry on with what you were doing. Hope this helps - as with everything the key is to keep it up at al times - Good luck.