Made an 18 month trip some time back. Lots of learning you will have along the way and most of the best tips will come to you as you make your way up/down the backpacker trail.
Some tips (by no means inclusive of everything)
1.) Try to keep some routines - helps you keep sane when you have those moments when you are tired, hungry, angry and have the crazy idea of stopping your travel. Remember, this is fun but there will be some bad days. Routines, like doing laundry, writing post cards and keeping up with your journal help in this regard.
2.) Don't marry yourself to a schedule or path of destinations. If you find a surprisingly good place, stay a while longer. If a place doesn't interest you as much as you thought, ditch it and move on.
3.) Without need to say, travel with other independents, couples or others going the same way as you. Backpackers naturally gravitate to each other so this is not difficult - 'safety in numbers'
4.) Be ready to travel your own way and break from groups if you want to go a different direction or different next destination. This seems difficult or rude at first but then you just get used to it. Remember, you are there to see the places that interest you, not someone else you met along the way.
5.) Go ahead and let mum and dad or someone else know where you are, where you are going every few days. It sounds silly but its best if someone back home knows where you are as you're moving around. In case of emergency back home, they can get ahold of you quickly. With internet being more widely available this is not as big of a problem anymore but you will likely find yourself in some places with very bad or no internet access.
6.) Buy snack food to take on long bus journeys. This sounds silly but you can starve when no edible food is found for 18-24 hours. Always nice to have a bag of chips/crisps or chocolates as emergency food.
7.) Have a pen ready at all times, incredible how much good information you get from fellow travelers about good hostels, restaurants, reliable tour guides, etc. Equally, they can share the bad ones as well. You need to get this stuff on paper, preferably in a notebook.
8.) Try to keep a budget, you seem to have a spend amount ready to go. This is good, try to average a daily cost and manage to it. If you go over a couple days, be ready to tighten your belt for the next couple of days in order to spread your costs out. Much easier to enjoy your trip when you can actively manage to your budget instead of finding out that you've exhausted your funds 3 months into a 1 year trip. This happens to a many!
9.) really think about souvenirs you may be interested in buying. Remember, you have to drag these things around for many months.
10.) Always look for power plugs so you can keep your electronics (especially your camera) charged. Sometimes you go days without finding reliable power sources so its best to charge whenever possible. Not really a problem in NZ or OZ but can be in Thailand, India and SA.
Ok, thats enough. Have to go to work now. Enjoy your RTW trip.