The most obvious way this problem manifests itself is in distractions. If you're spending 4 hours on Facebook or Twitter, or simply sufring the web for no particular reason, then you're gulity of allowing yourself to get distracted. You can't write while you're ready every status update posted on Facebook in the last 4 hours. It's just not going to happen. You have to learn to close your web browser and get back to writing. Better yet, don't open the web browser in the first place.
There are other ways that time management becomes a problem, at least for me. I'm an incredibly impulsive person and I get e-mails from clients all day long. I check my e-mail (which I should probably only do once a day anyway) and accept virtually all requests from my clients. Need a 2000-word article by noon? No problem. Generally. But when I accept 6 requests for articles to be completed by the end of the day, and lengthy articles at that, I might just have a problem managing my time well enough to get them all done.
Fortunately, solving time management problems isn't that difficult. You simply need to be able to see the projects you've accepted, their deadlines, and have them broken down into smaller tasks. You can do this easily enough with a large whiteboard or even an oversized calendar. Put it up where you'll see it constantly. When you start getting distracted or you're tempted to take on too much, just look up at your calendar and remind yourself that you have a job to do. Now. Not tomorrow.
I don't like whiteboards and my calendar is pretty and too small to help. But I do love post-its. I write tasks on post-its and plaster them like a calendar all over my window, right in front of my computer. This way, I can look up when I'm wasting time on Facebook and realize that I really do have to get a chapter done. Now! Then I can close my browser and get something done.
Admittedly, managing your time takes a degree of will power. But, if you're going to be a professional writer, well, you have to learn to discipline yourself and get something done. Get a lot of somethings done. It's the only way you'll make any money as a writer. Writing is a great career, but you have to work at it.