The one thing that actually worked for us, was giving high quality treats! Our 10 month old Maltese is trained to go indoors. He has a Pup Head fake grass square in the house that he goes on, which are great btw, for apartment dwellers.
http://www.portabl***gpotty.com/
Each time he did his business on the pad we would praise him and give him a treat. After that it just clicked with him, and he went on the Pup Head pad all the time. He loooves getting those treats! We give him Zuke's mini naturals in chicken formula. He perks up every time he hears the package, LOL. He has no problem going outside either, but it has to be on grass.