2. Give The Recurring Characters More Time To Develop
In addition to giving the narrative time to develop, multi-part stories also give the recurring characters (River Song, Madame Vastra, Jenny, and Strax to name a few) more time to develop. Even with original characters, like Warwick Davis's Porridge in "The Silver Nightmare," having more time with them will help them to stay with the viewer long after the end credits of the episode have finished rolling. Even if characters don't appear in consecutive episodes, there still ought to be a sense of growth between appearances. Take, for example, Craig Owens, who first appears in the season five episode "The Lodger." Craig doesn't have high aspirations for himself, stuck in the friendzone that his colleague and love interest Sophie had placed him in. We next see Craig in the season six episode "Closing Time," having worked hard to better himself, marrying Sophie and even having a baby with her. Contrast to Madame Vastra, Jenny, and Strax, the Doctor's Victorian era associates: although they appear in multiple episodes, there is never much developed in between appearances. Given more development, these three have the potential to rival River Song in their importance to the Doctor's life.