When shipping items make sure you have established the extent of the carrier's liability and take out any necessary extra insurance. Sending items by ship takes about a month, so try to arrange an address in Japan where you can send your things ahead of you. Bear in mind that the first and last few days of the month tend to be the busiest times for moving, so try to avoid them. If your things are due to arrive on a national holiday it will create delays, so check this before finalising your booking (in the section Work/Convention there is a complete list of Japanese Public Holidays).
If you’re packing your things yourself and you’re going to be away for an extended period give yourself plenty of time in advance to prepare. Plan how you’re going to pack, don’t just start randomly. An international move can be a good opportunity to get rid of unwanted or unneeded items – this will save you time and money when you come to ship your belongings. Make sure your boxes aren’t too heavy to lift and use proper packing boxes and strong tape to seal them. Label all your belongings with your name and a forwarding address. Clearly mark fragile items. Unpacking your boxes at the other end will be easier if you write their contents on them with a marker pen, or write messages to yourself like “open first”, “kitchen things” etc.