Take criticism. Believe in your work and stand behind it, but don’t be afraid to make changes. Try to be as objective as possible when it comes to your writing (I know how impossible that sounds) but it will only help you in the long run. Use rejections to evolve yourself as a writer, not just to line your waste basket. When someone rejects your work and offers a reason, don’t just blow it off and claim that they “didn’t get it” or that they clearly didn’t read it closely enough, dissect it and try to figure out if what they’re saying makes sense and if it will inevitably help your work. There a lot of people in this industry—agents, editors, other writers, etc.—who know what they’re talking about and know what it takes to make a book work. After all, that’s what they get paid for! Listen to them with open ears and grateful hearts. There’s a fine balance between staying true to your art and being open for suggestions, try to stay somewhere in the middle. If they “didn’t get it,” chances are, readers won’t get it either. And you won’t be there to explain it to them in the middle of Barnes and Noble.
Also…read, read, read! Get to know your genre. Regardless of how you feel about the books themselves, if you don’t know what’s selling, how are you supposed to write a book that sells? Believe it or not, publishers are in the business of making money. No ifs ands or buts about it. Sure, they’re passionate about literature but they’re more passionate about their bottom line. Because that’s what allows them to do what they love: publish books. I can’t stress this enough: You have to be familiar with what publishers are buying in your genre. You don’t have to be an exact replica, (actually I’d advice against that, it usually ends in a lawsuit) but you should at least know what else is out there.
And one final thing, on a more esoteric level: Be what you want. Don’t ask for it. Don’t hope for it. Don’t wait for it. Be it now. Being a “writer” is not something that happens when you sell your first book or land on a bestseller list. It’s a state of mind. It’s who you are. If you’re a writer, you know it. You feel it in your soul. So get out there and be it! When someone asks you what you do, say, “I’m a writer!” Not, “I work in sales but I’m trying to be a writer.” You are a writer now! I don’t care if you don’t have the check to prove it. Be it first. The money will follow.