Wow. I was just thinking about this for the past few days and discussing it with some people.
There are some things that I shall be keeping till after I die (if that is possible) but they are not the "usual" things (a stuffed toy, blanket my first high powered microscope, first plane ticket stub to China way back in 2002 that's about it). I've oodles of family heirlooms, which I cannot wait to give to my neices and nephews when they are old enough because I just don't want them. They really don't hold any value to me other than keeping my family quiet for taking them and stashing them in a box.
I've a rule of thumb, when clean through things, if I've not touched something in six months, I really have to think if I need it. If I convince myself I "need" it. It can stay. When I move, if I've not touched something in two years, I chuck it out no questions asked. I turn off the sentimental portion of my brain and toss.
When you globe trot as much as I have, you learn really fast that material objects are just materials, the memories are what make them special. All I have to say is thank technology for computers, internet, digital photos... soooo much easier to tote around a lap top than dozens of photo albums!!!