In The Murder of Roger Ackroyd, how did Christie prevent the reader from suspecting Dr. Sheppard as the criminal?

First and foremost, Christie took the unusual step of having Dr. Sheppard function as the first person narrator of the story. We see everything through his eyes, filtered in such a way as to make himself look innocent. He becomes Hercule Poirot’s confidant, chronicling the events and even adding his own suspicions to throw the reader off even more. Because he only speaks in good terms of the victim and is a respected member of the town, as readers we instinctively trust him as well. 
In addition, Christie also used her classic technique of having other characters with motive and opportunity. At one point Poirot observes, “Everyone at this table is hiding something,” and he is right. Dr. Sheppard acknowledges, “Everyone’s eyes dropped before him, including mine.” But we still don’t understand precisely what he means until the end. Christie leads us to believe he is feeling guilty about secretly hiding Ralph Paton in a hospital.
Ralph Paton becomes a major focus of the book, with many characters (including the police) speculating about where he is and whether or not he is the murderer. He seems to have motive and opportunity. As readers we also focus on Paton, with the other characters coming in and out of focus as their secrets are revealed. The doctor seems the most innocent of all, to the reader if not to Hercule Poirot.
In the end Poirot gives the doctor a chance to make his own choice, and Sheppard chooses to commit suicide, leaving the last chapter of his narrative the explanation of all the misleading clues and omitted information within the story. As it typical with Christie, the reader is left saying, “Ah ha! So that’s what happened!” and everything seems clear.