Registering in your home country is free and quick and it is the only way to make sure people know whether or not you want to donate your organs. Too many people when forced to make a rushed decision say no and this means that people who could have gotten a transplant die. Look at the info, discuss it with your family, think about what you want but most of all make a decision and make it known.
If you are australian: http://www.medicareaustralia.gov.au/public/services/aodr/index.jsp
If you are canadian: http://organdonations.ca/
If you are american: http://organdonor.gov/
If you are english: http://www.uktransplant.org.uk/ukt/default.jsp