The one I want to write about now is "Go with the flow," hardly writing-specific advice, but oft applied.
Here's how it's generally described: If, when writing, your scene takes you in an unexpected direction, follow it! Goodness may await you on the other side.
Here's how it's generally taken: The muses have taken over my brain and now my characters are on a zombie Zamboni and goodness may await me on the other side.
Here's what the original writers meant to say: So you're starting a story and have point A, which might be 'two characters meet for the first time', and a point Z, possibly 'they get together' or 'they fight crime.' In between them, you construct various other letters of scenes leading to plot; B might be 'they go to the zoo, and encounter romance/a robber.' If, in between writing A to B, your characters meet the mysterious stranger who entices them to go to her workplace, the crypt, instead, you may as well see where that leads!
But if you keep doing that, and Z is now 'they stop the ravenous hordes', you've lost the point of your story. Which is bad. Not that zombie stories don't have their place, but if you were planning to write a story about solving the Great Zoo Caper, then you're bust. If you now want to write the Zombies vs. Zebras story, you should write that story instead. But now you have new B, C, D, etc, and it is not the story you set out to write.
Which is all well and good if your zombie story is better. But if you're on chapter 4 and your characters just blew up the moon, then you need to stop following the flow and start making strategic dams.
My advice: When I was working on my latest story, I realized thanks to a helpful comment that I needed to have conflict between the two species in my story, the peasant humans and the noble, elf-like dzali. This conflict simply wasn't there when I started, but once I realized it, plots and themes just started dropping into place. It was a decision made without a lot of thought, but that had a lot of impact when it got moving.
But there were lots of times in that same story when flow burst out and drowned individual scenes. In one, the main character is looking for his friend in an army camp; he spins in a circle to do so. It seemed natural to me that something should happen at the end of that circle, and it would be too predictable if he found his friend. Instead, an upity dzalin orders him to help him in a task, carrying wood through the army camp. Heads begin to turn as people who know who this human is wait to see what he will do, and tension is building as they wait with bated breath to see when shit is going to go down.
And... I knew that my next scene was a bunch of people getting on a ship, and here he was headed the wrong way, doing something irrelevant that seemed significant, with someone who isn't important to the story. At this point, I have three choices: make this walk off stage a new basis for a plot, find some way for him to get to his new position carrying wood off stage to getting on a boat, or get rid of it.
Up I highlighted the last page I wrote, full of crowd reactions and characterization of the new, useless character, and pressed delete. There was nothing wrong with the scene, but it simply wasn't going to work with the rest of the plot, and I did not consider this nobody character, made up on the spot, to be important enough to derail everything. Instead, my character found his friend and went on with the plot.
So when someone tells you to go with the flow, don't just let it sweep you mindlessly into the rocks. Use it to power your story.Figure out where the flow is headed, and make sure that was the way you wanted to go in the first place, not just merely the one that seems good at the time.