One of the hurdles of writing dialogue for a story based in the past (in my case 1930s London) is that you have to keep reminding yourself of how people talked way back then. Everything from slang to formality of language has to be kept in mind — even for the simplest scenes.
So here is my Top 5 List of tips:
Read as much non-fiction as you can from the time and place you are writing about. This can include newspaper articles and magazines, but in the case of really really bygone eras, may include translations a limited number of books preserved from that period.
Read published speeches, honestly, though more formal than your dialogue is going to be, I find the way people wrote things that they then spoke aloud to an audience is a great source.
Read other writer’s fiction that is based in the same era (so people like us!)
Use the internet as a resource (obviously). I for example have two dialects to keep track of — Portia’s which is based out of 1930’s San Francisco and everyone else she interacts with (who are very much based out of 1930’s London). I like to use University Libraries, like this one to get ideas about how people wrote to each other and spoke about events happening at the time.
Watch movies from that era (if you can) or about that era (in the hopes that the filmmaker did HER due-diligance in researching the dialogue before committing pixel to film). For the 1930s in London, Wikipedia has a lovely list I have been culling through of films released in this time period. I find after watching one of these movies I am usually inspired to re-write a scene of dialogue to try and make it even more authentic.