For most of us, a round-the-world trip is a once in a lifetime experience. After all, it's not cheap! Still, it's important to think about the fact that you may only get to do this once, and you'll probably never visit most of these places again. So the last thing you want to do is come home and realize that you missed out on seeing things you really did want to see. That's why it's a good idea to do some planning before you leave so that you can maximize this great opportunity to see the world.
Remember, too, that although planning is important, so is spontaneity. There will be times when you discover something new, or an opportunity crops up, and you don't want to be so tied in to a schedule that you can't make a slight deviation from your plans. Try to maintain a balance - have an itinerary with a certain amount of planned activities, but gaps in there for when you just want to do something you weren't planning to do.
One of the first things you need to plan for is travel time. It's no good assuming you can get from A to B in a few hours, if the reality is that you need to catch two different trains and the schedules force you to take an overnight stay between them. Get yourself a bunch of good guide books, and read them! Your travel agent can also help you out here, as they should be familiar with many of the areas you're planning to visit. Sometimes, too, you can save time traveling overnight by choice. This can often be a good option when you have a long ferry trip to take.
Another good reason for reading a guidebook is to find out about local festivities that might clash with your visit. It's great if you're keen on visiting a festival that's happening at the same time as your stay, but it might also mean that if you think you're just going to wander into town and find somewhere to stay on the spot, you could be left stranded without a hotel room.
When you're planning how long to spend at each destination, make a list of all the things you want to see, and how long you think each one will take. Then allow for travel times, and you'll get a rough idea of how much time you need to spend at each spot. Remember, it's easy to underestimate how long you think you'll need, so be generous with how much time you allow. The last thing you want to be is tired all the time, running from sight to sight, and never really having time to relax and enjoy anything.
It's probably worth repeating that last point - schedule time to relax! It may sound silly to say you need to relax, when you're already going to be on a long-term holiday, but it's true. After 10 months of traveling around the world at a fairly hectic pace, I certainly was feeling quite tired and was getting to the point that I didn't bother going to see "yet another" waterfall. Sometimes you need to put your feet up, go nowhere, and give yourself time to take in everything you've seen. Maybe you've found a gorgeous view, or a particularly nice lake - allow some time to just relax and enjoy being in the moment, not worrying about having to race off to your next destination.
If you follow all these ideas and find that what you want to do isn't going to fit into the time you have available, you have a couple of options. Firstly, cut down your itinerary. That may mean visiting fewer places, or just doing a little less at each place you visit. Secondly, extend your holiday. Sounds like a good option to me!