I really try not to accumulate stuff, as I may have mentioned before. It's not that I'm not materialistic or anything like that - I'm naturally acquisitive, like most people, and, if there were no costs to it, I'd like to live in an infinitely large space with an infinite collection of things that I could access instantaneously.
But here in the real world, keeping stuff has costs. It takes up your living space. You can't buy anything because you don't have space to put it. You're afraid to move. You can't find the things you want among your possessions. Plus you should probably occasionally dust all that crap.
A few months ago, I went through all of my books. I kept the children's books that were in good condition because I might have a child. I kept non-fiction books that I enjoyed. I kept books I have not yet read but intend to read (of which there are only a few). I kept a very small number of books where the actual copy of the book has sentimental value to me. And I kept some books that I like to read over and over.
All other books (about half of all the books I owned) went to a thrift store. If I ever want to read one of them again, I'll be able to find it at a library. Meanwhile, I have a lot more space in my room and, as a bonus, someone else will get to enjoy those books for cheap.
I also went through my knick-knacks, of which I really own very few. Knick-knacks are dangerous because, the longer you keep something, the more meaning accrues to it, until suddenly that ribbon that meant nothing to you when you were 8 and got it for participating in a 1k run is something you can't bear to part with, just because you've seen it so many times. This can happen in just a couple of years. A mug from a job you hated will become a treasure because it reminds you of a prior era in your life. Throw it out early before it starts to accumulate significance.
Objects are not memories. I believe in keeping a real minimum of memory-objects. This includes family heirlooms. If a thing is useful to you, or looks good in your home, then keep it. But you don't have to keep everything your grandparents, great-grandparents, etc., ever owned just because they owned it. I feel like we should really fight the way that meaning accrues to objects over time, because it leads to owning way too much crap just because, you know, "Oh, that was the pan my mom always made brownies in." Get rid of it! Take a picture first as a keepsake. You can store a lot of pictures in the space it takes to store one artifact.
Another thing people keep is objects that might be useful one day. Say you bought a pickax a few years ago for a project. It's a pretty nice pickax, so you've been keeping it in case you ever need to use it again. Maybe you think you'll need a pickax once every 10 years. Is the cost of a new pickax in 10 years really worth storing a pickax for 10 years? Think about this carefully. Many useful things can be rented or borrowed when you need them. And there is probably someone else out there who really does need a pickax, and would be happy to have your old one if they found it at a pawn shop or thrift store. Pickaxes are made for use, not sitting around, and yours will thank you for getting it back into work rather than leaving it propping up your garage wall for years at a time.
Clothing is also good to consider giving away. How many t-shirts do you have, and how many do you need? Do you wear one daily and do laundry twice a month? Keep 15 or 20 of them, then, and get rid of the rest. If you're keeping them for sentimental value, take a picture instead. Do you have more than one set of clothes that you'd only wear to a funeral? Is this in case you have to go to two funerals on the same day and you get dirty inbetween? How about interview clothes? Do you have some items that you could, in theory, wear, except that they don't fit quite right, or for some other reason you don't ever actually wear them? Do you have clothes that are a size you probably will never be again? Are you hoarding these things because you're afraid they will stop making new clothes for you to buy if you need some in the future? Or is a hypothetical $200 you might someday have to spend on a few new pairs of pants or a business suit really worth storing dozens or hundreds of items for years?
Objects are not memories. Useful things can often be rented or borrowed. You don't need to own things you can readily get access to for free, or for very cheap. Storing items has a cost. Fight the tyranny of objects!