1. Write.  All the planning, thinking, dreaming, and research in the world won't get you a contract if you don't write the words down.  Make writing a priority.

2. Find a critique group or critique buddy.  Writing can be lonely, and a writing group can be an amazing source of support and inspiration.  They can also make you a much better writer.  It's remarkably easy to make stupid mistakes because your mind already knows what the words should say.  In this case many pairs of eyes are better than one. 

Critique other people's work to learn how to improve your own writing.  You'll be amazed at what you learn.  And develop a thick skin about your submissions—you'll need it.  If your critique partners don't catch something, your editor will.  It's easier on your ego to hear it from your crit partners first.

3. Learn from your critique partners, but also stay true to your own voice.  Everyone writes with a distinct "voice".  I suspect no two people would write even a simple paragraph the same way.  So learn from what your partners have to say—they have things to teach you, and their impressions as readers of your work are valid.  But also keep in mind that you are unique, and so is your author's voice.  Learn when changing your words would be an improvement, and when would be a detriment.

4. Learn the craft—and realize there's always more to learn.  Be a sponge!  Read, read, read—not just in your chosen genre, but everything and anything.  Watch films and television and pay attention to the scenes, story arc, and dialogue.  Study books that explain the craft and use them to hone your skills.  Learn from others' wisdom—it's much faster than trial-and-error.  And be prepared to discover how much you don't know, but thought you did!

5. Protect yourself.  If your critique group or partners aren't right for you, find new ones.  If family members or well-meaning friends don't understand your writing habit, sit down with them and tell them what it means to you.  Be kind and loving, and try to help them understand how much you enjoy it—but if necessary, draw your boundaries.  Be prepared to fight for your happiness and your sanity.  It's up to you to decide how much they're worth.  And treat yourself right.  You're the only person who can ensure that anyone does.