Scratching at the door seems to be an innate behaviour for dogs. The dog wants to come in the house and needs to get your attention, so they do the obvious and scratch the door. The easiest way to prevent dog scratches on doors is to recondition the dog not to scratch the door. There are many different products on the market to facilitate this transition, but your best weapon will be to recondition the dog so that the dog does not associate scratching the door with a positive outcome.
Determine the reason that the dog scratches the door. Is it to receive attention, burn excess energy or to communicate that the dog wants to come in or go out? If it is to let you know the dog wants to come inside or go outside, you can install a "doggie doorbell" and teach the dog to rattle the bell with his nose instead of scratching the door. You can design a dog doorbell yourself by attaching bells to a long strip of cloth. If it is to burn excess energy or to seek attention, you can try ignoring the behaviour. Often dogs perform a behaviour as a means to get attention.
Install plastic sheeting over the areas of the door that the dog scratches. You can buy a plastic door shield from pet supply stores.
Work with your dog to avoid rewarding attention-seeking behaviour such as scratching doors. Dogs often scratch doors due to separation anxiety. You can try behaviour modification exercises such as rewarding the dog using treats and attention only when the dog is calm. You can also desensitise your dog to departure cues by performing activities such as putting on your coat and picking up your keys several times a day but not actually leaving the house.
If the dog scratches at an internal door, build a booby trap that creates an unpleasant noise each time the dog scratches the door by tying aluminium cans to a hanger and placing the hanger on the back of the door. Installing a dog door or confining your pet to a crate when you are not home will cut down on door scratching. If nothing else works, you may look into buying a mat that discharges a mild static shock each time the dog contacts the mat; place the mat near the door that you want the dog to avoid.