I remember two pieces of advice I have gotten that impacted me a lot.
The first was from my dad: "Write like everyone who reads this are stupid." Might sound pretty harsh, but for child-me that had poor sense of theory of mind (basically knowing that other people aren't mindreaders) it made me remember that people can't see what I think, so I had to me more clear in my storytelling.
"Write the story you want to read." I've seen this a number of times around the internet. I think I first heard about it from a Danish author I adore though.
"Never have two heads be the same size in on a page" This small advice has made my pages look a lot more dynamic and interesting. If I notice I have two heads the same size, I deliberately scale them op or down to avoid it.
"Done is better than perfect" I have a sticker om my laptop saying this.
And not so much an advice as a hack I found for my cintiq: Buy a small plastic ruler, that makes it so much easier making straight lines digitally. Because ofc a ruler helps on straight lines!