For me there is only one rock-solid rule I refuse to break. Whatever I write, I strive my hardest to make sure my reader understands what I write. I look at my writing like painting a picture. I want you, my awesome readers, to see the same picture I'm painting with my words. Something I've apparently managed to miss in my latest book (according to the reviews). It seems I was lacking getting across just how the magic works in my world.
I've always said feedback is invaluable, but helpful feedback can be hard to come by. That is why I comb through my work several times just to make sure what I've written paints the same picture just as clearly as I want.
The last time through is read to me by my computer. My computer might spell out the occasional odd name, or pronounce something that's supposed to be spelled, but these can be easily overlooked. Where it helps the most is finding those small spelling errors my brain refuses to see, or the missing endings my brain always adds because I want them there. It is also a tremendous help in the placing of the illusive comma.
Occasionally, on Facebook, someone will post something showing the tremendous misunderstandings that can arise from the lack of proper punctuation. The latest one was a magazine cover listing some of the articles on the cover. Without a space between those titles the magazine had someone cooking and eating their dog and cat (sorry I don't remember the name - it was about a talk/cooking show). There are many examples, and generally they are certainly worth a chuckle, but it's not something you want in your book. I read one place where one person refused to use any commas - he didn't understand their placement and never got them right. Whatever rocks your boat, but I won't be reading any book he writes, though I might struggle through a blog post or two.
Fortunately, the very nice computerized voice of my computer helps me with comma placement too. The old rule, if you need a spot in your sentence to breathe, put a comma there, but it's more than that. Sentences have a rhythm, and commas help foster that. Even though the comma is a subliminal thing, our brains need that moment to breathe. Without it, the run-on sentence risks confusion, and you never want to confuse your reader.
I've been told I use too many commas, but I've also been told that my reading is very easy and quick. I think, if the reader doesn't have to struggle with their own punctuation and breathing, the writing is smoother. At least that's what I strive for. There's also a possibility that my writing is too simplistic, though both my boys say it's too complicated. Now you know why you have to pick and choose what feedback to take to heart, though whether my stories are too complicated or too simple is something I can't do much about. I try to tell a story that is clean enough for your kids to read, but I want a story adults will enjoy too; I'll leave it up to you to decide if you want their kids to read them.
So how many rules do I break? I have no idea - hopefully not too many. How many rules do you break?