When we made the decision to move across the country the first of two times, we were living in a 530 square foot apartment, so downsizing wasn't too terribly overwhelming. Limited space = limited stuff.
That being said, we wanted to be very selective with the items we chose to keep because they would need to arrive on the West coast in one of two ways: 1) packed in the Honda Civic's backseat or trunk to ride along with us on our cross-country moving adventure, or 2) shipped via UPS. Option #1 offered limited space. Option #2 was a cost issue. In either case, the importance of paring down belongings was a critical component of this move.
About six months prior to our departure we started sorting and purging items in order to give ourselves plenty of time to sell what we wanted to sell and ship what we needed to ship. Items that made the cut and rode with us in the car included a durable file box with all of our financial paperwork and important documents, two bicycles, and our cat. Items that also made the cut but arrived via UPS included about half of our clothing, various memorabilia items, art supplies, paper shredder (don't ask me why we chose to keep the shredder instead of purchasing a new one after the move), blender and silverware. Items we sold before we left included all of our furniture, cookware, dishes, glassware, many books and half of our clothing.
Eight years have passed and another cross-country move has occurred. We now live in a 700 square foot condo where we continue to edit and downsize, and funnily enough we still have the paper shredder.