1. Make sure your book is unique and that there is genuinely a gap in the market for it. For instance, I wrote a non-fiction book from a Christian perspective on the topic of dealing with illness and seeking healing. While there were plenty of books on helping people process why bad things (such as sickness) happen and there were plenty of books teaching well people how to pray for those who were sick, there wasn't a book that I could find that guided an ill person in how to prayer for healing. And there certainly wasn't a book that balanced this against taking medicines and going to doctors.
2. If you are a first time author make sure you get an agent. Publishing houses very rarely engage with authors directly, and certainly not if you don't have a proven track record. A friend of mine advised me to get an agent and I wouldn't have got my first publishing contract without one.
3. Get honest feedback early on. My book is so much improved by all the input I have received from friends and family and people in my field, but in particular, before I even submitted the concept to an agent I got three friends to look through my sample chapters and they helped me see what worked with my writing approach and what didn't. There is no such thing as bad feedback because even if you completely disagree with the feedback you have received it is helpful to know that some people see it that way. Negative feedback broadens your perspective and makes you more aware how your writing can be interpreted by different groups of people and it makes you a better writer. It can be hard to take but it is always useful.