Recently, this lovely black Lab helped me demonstrate how to attach the world "heel" to a specific behavior. Notice that I work her near a wall or a piece of furniture. Why? I want her to walk in a straight line next to me and sit next to me without swinging her butt out. Also, each time we stop, our toes should be aligned.
Practicing lots of off-leash heeling indoors will soon have you walking your dog through your neighborhood, on-leash, at heel - making you very proud and your neighbors very jealous!