Take a quick spin today through at least one of your best spots to store things – the attic, the basement, way back under your bed – and see if you can spot anything that you can now look at and say "Why on earth am I still holding onto that?"
Sell it, give it away, or put it into the bin for the trash/recycling angels to remove from your life.
If you can get into the habit of doing this every month or two, your next move will be much easier (and if you never move again at least your heirs won't be sobbing "What is all this crap?!")