Today we are talking hindsight. The things we wish we knew before we began. There’s really only one thing that I can think of, and it’s not really a before I started writing type of thing.
Editing. I learned so much through my first round of editing, that I wish I had done it before I submitted anything to any publisher. So what I would recommend is if you have the money, speak to an editor. Get someone to look over your work and give it a professional edit.
Not just a read through from a friend to tell you what they think. Or even a proofread for grammar and spelling. A full, in depth look at everything. Context, wording, character, plot… anything and everything. Because once you’ve had a professional edit, and seen those little things that everyone learns the first time around, you will be better set for future works.
And on the plus side, you’ll have an edited manuscript that is in even better shape for submitting to publishers!