FWIW - For those who have unwanted objects that you can't let go of for 'sentimental reasons' - I've found that the sentiment is that the object allows me to access memories, I wouldn't otherwise remember, even if I tried. Often just seeing the object in question triggers the memories. So, one thing I've been experimenting with, is taking pictures of the objects and storing the objects out of sight. Periodically, I flip (digitally, of course) through the objects, 99 out of 100, I remember precisely the memory I wanted. For the 1 in a 100, it's usually something else about the object (the smell, the texture, etc) that triggers the memory. In those cases, I simply store it in a box with the other 1 in a 100 items and get rid of the other 99 items, where I've 'transferred' the sentiment to a digital image.
It is a hell of a lot easier to carry around a terrabyte hard drive than it is to carry around a storage unit full of crap.
Someone mentioned up thread a wife who can't let go of baby clothes for a kid who is 19 now. Try taking pictures of the various outfits and making a photo album (physical is best in many ways) of them and give it to your wife. For her, it may be that an outfit triggers a very specific memory of your child when they were growing up. Some folks have a very hard time letting go of memories and/or finding them again.
I think that's something that we have let go of, in our digital age, is photo albums and the importance of maintaining an archive of memories. I'm not saying everything has to be photo-fucking-documented. But taking the time to curate memories and maintain them - can and does make it easier to let go of material objects.