"Leave it" is a handy behavior for dogs and owners, and once learned, it can help keep a wide variety of inappropriate things out of your dog's mouth! Best of all, it's pretty easy to teach using positive reinforcement.
One additional tip: Leave It and Drop It should be different. One (usually leave it) should mean, "Don't put that in your mouth," and the other (usually drop it) means, "Now that it's already in your mouth, please give it back." It's important to remember the difference so as not to confuse the dog.