To really and truly write what's in your heart. If your'e not having fun writing it, no one is going to have fun reading it. Remember that absolutely anything can happen in fiction; don't get tied up in mundane details, go wild. Then remember that fiction has to make more sense than life. :)
Also, develop a thick skin if you're going to be submitting your work because there is a lot of rejection out there. Just keep trying and learn from your rejection letters!