Know what you want to write about in detail before sitting down in front of your computer or notepad. Staring at a blank state can be paralyzing. I never start a new project until I have the storyline etched out in my head with at least two plausible endings. Not one of my books has ended the way I thought it would, but I would not feel comfortable diving into a story and spending months writing only to find that I had no satisfying way of ending it. But I also recognize that once I create my characters, they take on a life of their own and will not do anything that doesn't jibe with their personalities. So, my advice is spend time learning about your characters, the story you want to tell, and let the actors play their parts as they see fit.