Feels like everyone's advice goes straight to structure or marketability, but for a debut, what made my manuscript stand out was ruthlessly cutting anything that didn't serve my protagonist's voice. I'd read chapters aloud and if a line sounded like anyone could have written it, I'd scrap it. The voice became so distinct, an editor told me later that's what got them past the slush pile. It's not just about a clean plot; it's about a personality that leaps off the page and insists on being heard. That kind of authenticity is impossible to ignore.
Workshop groups are essential, but you need the right kind. I found writers who were better than me, not just supportive friends. They tore my darlings apart, and I'd go cry, then rewrite. The script went from a 5 to a solid 8.5 through that sheer, brutal iteration. Submission is a numbers game, but you only get one shot per agent with a project, so make that shot the most polished, unmistakably you thing it can be.