1. Book your flight(s).
If you want to book multiple flights in advance, like we did, then it's a good idea to go through a travel agency as they can tailor make your flights using the airlines with the best deals. STA is a popular choice, but we actually found a cheaper package with Round the World Experts.
If you want to just book your first flight, so you have maximum flexibility on your trip, then I suggest you use price comparison sites (such as Expedia and Skyscanner) in order to find the cheapest flights possible.
2. Sort out your visa(s).
Even if it's only for a short period of time, certain countries (such as Australia and Vietnam) will only permit entry if you have a granted visa. Use Government websites to research, apply for and obtain the correct visas before you go. It's fairly straightforward and one less thing to do while you're out there!
3. Think about daily transportation.
You've booked your flight(s), you've obtained your visa(s), but how are you going to travel around once you get there?
It's no secret that it's easier (and cheaper) to use local methods of transport in each country - like trains, buses and even ferries. Some countries also have coach providers specifically for tourists and out-of-towners with hop on/hop off passes and different routes which will help you cover a lot of ground on a tight budget. For example, check out Stray or Kiwi Experience for New Zealand, or Greyhound for Australia.
4. Get the relevant injections.
This is a very important one, as you don't want to be struck down by a life-threatening illness when you're travelling. Protect yourself by getting the required vaccinations. You can book an appointment with a nurse at your local GP for this. Another option is to visit a travel clinic (such as STA Travel Clinic), but these will generally be more expensive.
5. Book some hostels.
For every flight that you've booked in advance, it's a good idea to research and book some accommodation for the night upon your arrival in the country. It goes without saying that you will be jetlagged and just need somewhere to crash temporarily, pronto, so try to arrange a hostel or hotel that will pick you up from the airport as part of the package. The Hostel World site is a great resource for this.
6. Sort out your finances.
Again, another important one, as you're not going to get anywhere without money! Move the money that you've saved for travelling into your current account, so it can easily be accessed when you need it. Then investigate getting a travel card, where you can use their website or an app to upload money as you go along, as you won't be charged using these abroad like you usually would with a debit or credit card. My Travel Cash is a good provider to use, but make sure you also take a credit or debit card in case of emergency.
Also, get a little bit of cash converted to the local currency just before you leave, so that you have some money on you in case you need to get a taxi from the airport upon arrival.
7. Think about your phone options.
If you want a phone for just calling or texting home, a lot of people suggest taking a Pay As You Go cheap phone that you can top up as you go along. I do understand the reasoning behind this, in case your phone gets lost or stolen. However, it is also a good idea to take a smart phone as a lot of places have free wi-fi these days, which means you can use Facebook and email to stay connected with people (for free)!
8. Make a list of everything that you need to take.
There are some pretty extensive ones currently on the internet. I personally like Wordly Nomads his and hers backpacking lists as these helped us a lot. One of the most important things you can buy though is the actual backpack. Make sure it's sturdy and of good quality as you will be taking it everywhere with you! Also make sure you have enough space for everything that you need. 45-60L is recommended for a three month trip, similar to ours.
9. Photocopy important documents.
Make sure you make photocopies of all your important documents before you go and keep them in a different place to the originals. This is worth doing in case something happens to the original documents.
10. Use Lonely Planet books to help.
Finally, it's a good idea to research the place(s) you are going to before you leave for your trip. It's a well known fact that you meet people along the way in hostels or on tours, as well as locals, who will help you get your bearings while you're out there. However, it's good to have a bit of a heads up beforehand so you know what you're getting yourself into! I recommend getting a Lonely Planet book for the place(s) you're going to, which are filled with answers to every possible question you can imagine.
Also, it goes without saying, ask around if you know any friends or family that have been travelling. You need all the help you can get, especially if you're going to multiple countries!