Most of us, after a while, decide on what rules work for us. These may change over time; a writing rule is only useful as long as it is helping you achieve your writing goals.
One of my writing rules is that I'm not allowed to go back and edit what I've written until the first draft is finished. This is a rule I had to adopt during my first NaNoWriMo, and I've never looked back. I've found that without enforcing this rule, I don't finish my first drafts. (YMMV, obviously)
A few weeks ago I wrote about my experience getting my groove back by participating in #AHundredOrDie on Twitter. I was having a great time, plugging along a little bit every day, until... I wasn't.
It took a lot of pondering, but I eventually realized my frustration was because of a structural mistake in my book: there was way too much time between the reveal of the twist and the characters acting upon it. I couldn't move forward without fixing this big mistake, so I broke my own rule and edited to get the scenes in the right order.
Progress restored! I was able to move forward. And that's the plan: continue moving towards The End without stopping to edit. Editing was helpful in the moment, but it was only an exception to the rule. Full steam ahead!
Which of your writing rules have you had to break, and why?