1. Learn to expect the unexpected. Life throws you in crazy ways normally, but being a TCK creates so many more opportunities for chaos. One time, my family was checking in for a flight at the airport, and we found out it was delayed. This isn't normally a problem, but we had a connection that we were going to miss because of that first flight being delayed. Our bags had already gone through customs, so we had to sit in the check-in area for 6+ hours! We couldn't even eat a meal. So even though you cannot expect the unexpected, just be prepared for disappointments and lots of change.
2. Friends change a lot. And I mean a ton. You move. People move. And other people come into your lives. Growing up in general is hard, but with friends moving in and out of your life, it feels like a lot. I know, I've been through that. A way to cope with the stress of losing friends is to be friendly with everyone, even if you're not that close.
3. Keep a journal. You will have so many cool stories from being a TCK and travelling the world, that it's good to get them down on paper. So many times, I will read back through my journal and laugh out loud. Journals are a great source of input when you feel like no one relates with you, too.
4. People will assume things about you. TCKs live lives that are very different to many people, whether in your host country or home country. People in your home country may assume that you ride a camel to school. Those in your host country might think that you are the British ambassador's kid, when you are not. Both of these things have happened to me! When people do make wrong assumptions, do not judge them or be rude to them because of what they say, it will only result in more trouble.