I just bumped into an excellent post over at Productivity 501 that asked this very question with answers from some of the most organized people on the planet.
The biggest time wasters they mentioned that really hit home with me were:
Watching TV. Y'all know how I feel about this. I go back and forth myself--sometimes I'll forsake TV completely for a year and other times I'll catch myself spending an entire Saturday (it seems) watching HGTV. (HGTV and the Food Channel are my Kryptonite.) But I know for a fact that it's a waste of time, and that when I'm disciplined enough to not turn on the TV that I use my time much more creatively.
Not having a game plan for the day. This may be just me, but on the days that I wing it I can start out doing just one little thing for fun thing, and then an hour or so later I still haven't officially started doing my real work. I find that having a plan (1. do this first 2. do this second, etc.) helps me stay on track and not get distracted.
Procrastination. We do this without even noticing it sometimes, and the reason why we don't take notice of it is because what we're doing to procrastinate feels so much better than the option of actually doing what we need to do. All of us procrastinate from time to time, and one thing I've found that works for me is to do something, anything to get the ball rolling. I ask myself "What are the first steps I need to do to get this done?", and then I force myself to do the first step, and then the second, etc. If there's something I've been procrastinating on, I'll also get all hard core and force myself to do it first thing in the morning, so I'm not dreading it all day. Some folks call this "doing fear things first", and I've found that it's uber helpful at decreasing stress.
Thinking about unimportant things. I love this one, because it's something that I think a lot of us do that we don't even notice. Have you ever had an argument with someone in your head when an argument hasn't actually taken place in real life? Have you ever replayed vexing conversations with people you'll never see again? Most of us do this without realizing that we're taking up valuable moments of our life on things that don't really matter, and when we force ourselves to become more conscious of our thoughts we'll gain valuable time.
Being busy without really accomplishing anything. On the internet we have so many tempting avenues for busyness, and I admit that I fall prey to them too :-). Playing around of Facebook, IMing, Twittering, prolonged blog surfing, etc. I know that if I don't watch myself a whole day can be sucked up doing these busy, but unproductive tasks. This is where it really comes in handy to have a game plan for the day. I have made a deal with myself that if I get all my work done, then I can take an hour and do whatever I'd like online. But you know what? After my real work is done, the temptation to do all the distracting internet-y things fades away. Could it be that busyness is another form of procrastination? Hmmmm... :-)
And I'd like to add 2 more I've noticed: