There are only three steps to remember in housebreaking any dog regardless of its age.  

Step 1, TAKE ACTION:  Don't wait for your dog to ask to go out and don't assume that he understands why the doggie door is there because he doesn't. Your dog doesn't know that it's not OK to soil inside the house.  It's up to you to teach him.  So, stop what you are doing and take your dog out at regular intervals. 

Instead of putting your dog outside by himself, take him on leash to the exact same place in your yard each time.  This will prevent him from 'forgetting' why he is outside.  Praise your dog lavishly, play with him, or offer him a treat as soon as he is finished.  Let your dog know he has done something fantastic.

I like to put a word with the deed.  This saves time and teaches your dog to go on command when and where you tell him to go.  You will appreciate this in bad weather or when traveling with your dog.  It doesn't matter what word you choose as long as you use the same word every time.

Please remember, if the dog has an accident after you have just taken him outside, it is ALWAYS your fault and never the dog's.  So, don't punish your dog for having an accident.  This will make him fearful of going in front of you and he will seek out places inside the house where you cannot see him.  

Step 2, SUPERVISION:  When you cannot supervise your dog, i.e. watching the dog instead of TV, or making dinner, or putting the kids to bed, or allowing the dog to leave the room and wander down the hall into a bedroom, tie your dog to you on leash or place your dog inside his crate.  Dogs consider their crates their dens and they do not want to soil their dens.  See the page on Crate Training for more information.

However, it is important that the size of the crate is appropriate for the size of your dog, which means that you might need to buy three different sized crates as the dog grows depending on how large the dog will become. 

The theory behind using a crate to housebreak your dog is that your dog will learn to hold it (within reason) if he knows he must lay in it.  Therefore, the crate should be large enough for your dog to stand up circle around and lay down again.  If your dog can eliminate in one end of the crate and lay down in the other end, the crate is too large and it will not facilitate housebreaking. 

Be careful not to leave him in the crate too long.  Allow him plenty of time out of the crate.  Also, the crate is not a punishment -- it's his sanctuary.  Be sure your children learn to respect your dog's privacy when he is inside his crate.  Don't we all need a place all our own?

Step 3, SCHEDULING:  It is very helpful to keep a written schedule of your dog's elimination times.  This will help keep you on track.

As a general rule dogs need to go out after eating, after playing, after sleeping, or after chewing on a toy or bone, or every two hours especially puppies and dogs that are new to your family.  

That's all there is to it.   We highly recommend these two housebreaking videos from PetVideo.com and PetVideo.com