Donald Maass says there should be tension on every page. You should print the book out, scramble the pages so that you’re not reading in sequence, and then make sure that every page has some bit of tension on it. I don’t do the scrambling or printing out bit, but I’m always trying to twist the screwdriver embedded in the characters’ backs just a little more.
I found though that my writing picked up when I realized that beyond tension on the page, every scene had to have one moment with an emotional high or low point. I found that every day, I write at least one scene as a whole unit, with one point at which the characters have that moment of climax (sometimes literally, though sex is hard because it releases tension and the whole boint of writing is bottling it up, so writing sex *and* mantaining tension is a bit of a foxtrot).
I don’t diagram my stories, but This person did. I work on a “if they have time to lean, they have time to scream” plotting structure where if I notice the characters have been sitting around talking, I throw a grenade into the mix and let them scramble. The biggest problem I have with most stories is the fact that so little happens over so many words, and making sure there is always something happening can help keep the story going. Being aware of how the emotional flow of the story is the first step having that be just part of your natural writing flow.