1) Read. Read the best of everything in your genre, and stay current. People often make the mistake of thinking “I was a kid once, and I know what I liked” or referencing books from their childhood, but children’s publishing has changed dramatically in the last 20 to 30 years. You have to know what the market is like today, and stay plugged in as it evolves – no matter what genre you write for.
2) Hone your craft. Take classes and workshops, attend conferences. Keep stretching, learning, sharpening your skills – even (or maybe especially) after you’ve sold your first manuscript.
3) Find community. Writing can be a solitary business. I’m lucky – I write with a partner and work for a graduate writing program, but it’s really important to find your tribe and connect with them regularly. Find a supportive critique group, join forums, take classes, attend conferences, whatever it takes to connect with other writers. It will keep you sane, and honest. (Start by joining the Society of Children’s Book Writers and Illustrators.)
4) Diversify your strengths. It’s the rare writer that makes a living solely from writing. Even the most successful writers in the world have to augment their income with things like teaching, editing, or speaking engagements. Find ways to support your writing habit. Be willing to have a day job, to do whatever it takes… but whenever possible, try to make those other sources of income writing-related, such as freelance writing, editing, teaching, etc. It really helps.