Lady Macbeth basically uses a four-pronged approach to wearing down the last bit of morality in her husband.
First, she asks him why he broke the promise to her? Of course, Macbeth never did actually promise her; he just sent her a letter detailing the prophecies of the witches. However, this does not seem to matter. Further more, she notes that he must not lover her, saying "From this time / Such I account thy lov." (I,vii).
Next, she asks if he is a coward. Typically, men do not like to being called scared, especially visious warriors like Macbeth. She claims
Finally, she tells him that she would do if for him, if she had promised. She uses the gory image of bashing a baby's head, saying she would if she had so sworn.
All in all, Macbeth is no match for his wife and gives in.