My dogs had a similar issue (several actually) when I rescued them and I broke them of the habit very quickly. What kind of dog?
What works really well for me is rewarding correct behavior, and giving corrections for unacceptable behavior. If I'm walking my dog and he goes to chase a rabbit I tell him "no" or "leave it". If he continues to give attention to the rabbit or to go after it I give a firm correction (a sharp tug on the leash to snap him out of it).
As for the scratching of the door area, you need to catch the dog in the act otherwise it's pointless and confusing for the dog. If you're not going to be next to the dog when he scratches, I'd get an E-Collar. Amazon sells cheap models for $12. These have different settings (Sound, Vibration, shock). I'd just use the sound/vibration settings. When you hear the dog scratching you give a vibration that snaps the dog out of it. Pretty soon the dog associates the scratching with the unpleasant vibration and doesn't do it anymore. Lastly, make sure you're around to praise the dog and reward with treats when he walks up to the door and waits.
One other thing, it may be a good idea to redirect this behavior to a bell. My latest puppy rings a bell by the door when he needs to go out. It'd be easy to teach a dog the scratches to start using the bell instead. If he scratches you give correction, if he rings the bell you reward and praise.
There's a really good message board on dog training where you'll get a lot of helpful responses from people who are REALLY into dog training. If you get a chance it's definitely worth reading.
www.leerburg.com