Write about what you know takes on new meaning when you're trying to narrow down all of your book ideas into one that will be worth it, and marketable. Though the ideas may come from all over and flow easily, you'll need to develop the ability to push yourself to narrow things down and work with only those topics that have a chance at publication.
So you have your idea narrowed down, now what. How much do you know about the topic really? Are you an expert or just an enthusiast? Even as an expert on your topic, you might find yourself needing to do research to round-out your idea and give a sense of realism to your work if it's fiction. If you're writing non-fiction, the research is a necessity. And not just research on your topic - you'll need to research your market, self-publishing or traditional publishing depending which one you plan to use, and how best to go about promoting your book once it's up and running.
Whether you're a fan of the outline or not, you need to have a guide for your book. Answer the questions you know an editor would want to know. You can even type it up in book proposal format. Get the particulars down so you know where you're going with the story and how you want it to sound and look. A journey of a thousand miles begins with a map.
Do some mental gymnastics to psyche yourself up. It can be a little depressing being a writer. Though I think we can all agree that the solitude part is epic, it also means that there's no one to pat you on the back for a job well done, or give you the much needed pep-talk midway through the tough parts. You have to be your own validation and cheering section, so don't skimp on the praise when you get stuff done and make sure you factor in rewards and down time (not too much mind you or it becomes procrastination). You opted for this gig because you thought it would be fun, so if you're not having fun as a writer, you're missing the good parts. Enjoy yourself and believe you can do it and do it well.
Write daily. It doesn't matter if it's any good, you're going to edit the living daylights out of everything no matter how great you think it is initially any way, so get the words down on paper. Even if all you have is 15 minutes to write, use it, sit down, and write. I think this is the area that gets most writers. I know it's where I stumble and fall more often than not. I have a friend who gets up in the morning and sits down and writes. Whether he feels like it or not, he writes. Every day. First thing. And he has several books published, go figure.
Lots of us (me) procrastinate though, and do yoga, or meditate, or get the kids off to school, or hit the snooze a million times to avoid our 'day-jobs'... anything except write. It gets rationalized with promises to 'do it later when the day calms down', etc. Maybe some days you will do that, but I think most of the time it's not going to get done. So if you can, if you want to be a productive writer, if that is your main goal – it has to be the very first thing you do when you get out of bed. After setting up that caffeine-fix of course.