7. If You Need To Link To A Larger Story Arc, Be Very Subtle About It
Story arcs are hard things to get right in a show like Doctor Who. If you set up the finale too much then the finale will inevitably disappoint. If you repeatedly introduce the narrative threads without development then the audience will lose interest. Therefore, the best way to handle story arcs is to subtly weave them in. It works best if we barely notice them at first, like with the Bad Wolf threads in Season 1. Alternatively, its also effective to use the Chekovs Gun: introducing an apparently insignificant plot point that later has greater importance. The Almost People did this brilliantly, taking the basic premise of the Doctor wanting to scan the flesh and turning it into a key revelation in the ongoing story arc of Season 6. Either way works, as long as it is sufficiently subtle to take us by surprise and warrant your episodes significance.