When you’re writing a story that’s going to take more than one session of writing, be sure that when you put off writing for the day that you end with a sentence that you can easily follow when you come back to write some more. Sometimes I pick up my pencil and I find that I can’t remember my train of thought.
Leaving notes for myself doesn’t help because even if I know where the dialogue is supposed to go I might not be able to remember what to write to make it seem natural. Let’s say the characters are talking about trains and the conversation is supposed to lead to one character inviting someone out for a game of Pool. If I leave off after the bit about trains, how am I supposed to steer the conversation to a game of pool? I’ll have to think up a way and that takes time.
Make sure you leave, say, after a particular scene is done, then maybe leave yourself notes on what the next chapter or scene is going to contain.