Instructions
Teach your dog a "Leave it!" command. Use this command at the moment your dog is attempting to chew an object that is not permitted. Shout this command loudly and assertively. Your dog will eventually learn that chewing is not acceptable and that it comes with consequences.
Walk your dog twice a day for 20 minutes. This will help to stop chewing out of boredom or restlessness. Your dog will have burned off a lot of excess energy and will be too tired to need any more stimulation in the form of chewing.
Arrange for someone to be home more often. This will prevent your dog from chewing because of loneliness or separation anxiety. If it's not possible to have someone home more often, crate train your dog. Crate training will prevent your dog from having access to power cords when no one is home.
Limit the amount of time your dog is afraid and monitor her when she is. As an example, if your dog is scared of thunderstorms and chews when they are occurring, remove your dog from any location with power cords until it is over. Limit your dog's exposure to all of her phobias by preventing her from accessing them in the first place, as best you can.
Provide your dog with acceptable toys to chew on. Teach your dog that it is okay to chew on these toys by offering treats and praise when she does. Use this in conjunction with your "Leave it" command to provide guidance to your dog as to what is acceptable and what is not. Chewing on these toys will help to satisfy her urge to chew -- and save your power cords.