When chewing becomes destructive
Dogs use their mouths in much the same way we use our hands - as tools for exploring. This means they examine anything they find interesting by chewing and gnawing it. Unfortunately, with some dogs this chewing becomes destructive and a problem.
Why do they chew?
Most puppies will chew when they are teething. As they become older they may chew on something they're playing with, out of boredom or from an instinct to chew. Chewing may result from a dog being in unfamiliar surroundings suffering from separation anxiety. Identifying why your dog is chewing is the key to stopping the behaviour.
If your dog is chewing on a wide variety of objects in the house regardless of whether you are there or not, he's probably chewing out of boredom. Your dog probably needs more play, exercise and physical activity to work off some of that excess energy. Providing appropriate chew toys will also help your dog kick its destructive chewing habit.
If your dog is teething it's something you're going to have to wait for him to grow out of. Whenever possible you want to redirect your teething pup's chewing to appropriate objects. Try making some frozen treats in popsicle moulds or leaving some of your dogs toys in the freezer before giving them to your dog. This will not only provide an appropriate toy, but one that will be soothing on your dog's teeth as well.
Some dogs chew as a result of separation anxiety. They may be chewing or pawing in an effort to escape from the area they've been left in. They may also chew on inappropriate objects as a nervous reaction to your absence. This is probably the most difficult type of chewing to treat because it happens when you're not present and it is the anxiety itself that needs to be treated. Dog behaviourists will use socialisation, desensitisation and counter-conditioning techniques to cure separation anxiety.
Timing is everything
The only time you can effectively discipline your dog for chewing is when you catch him in the act. If you find a chewed up shoe an hour after your dog has done the deed he won't be able to associate any punishment with the act of chewing. Your dog may seem 'guilty' when you wave the shoe in front of his face but this is just submissive behaviour. When you do catch your dog chewing, stop him with a verbal command or a loud noise. When he stops, give him his chew toy. When he starts chewing on it, praise him.
For most types of chewing you should help your dog by redirecting him to chew on appropriate objects. Choosing the right type of chew toy for your dog is the first step. Never give him anything that looks like something you don't want him to chew on. If you give him old shoes or clothes he won't be able to understand the difference between them and the items he's not supposed to chew.
There are some products that can be applied to household objects to discourage your dog from chewing them. These products have a taste or smell that is unpleasant to dogs but inoffensive to humans.
As always, consult your veterinarian. They'll be able to provide useful advice and suggestions.