PTF, primarily.
I might be moving soon, which is excellent inspiration to start getting rid of things.
I live in a shared house --- so pretty much everything I own is stored in my bedroom which has a small closet, a bookshelf, and a night stand with drawers. And, yet, I am SHOCKED at how much I've accumulated in the three years I've lived here.
Also, my room is a ridiculous mess. Because I am lazy. Which is totally a life fail. I've been saying that I would take a weekend day and do a full-on Marie Kondo style purge of everything.
But, that hasn't happened.
So, instead I put a trash bag and box for donations by my door. And I'm trying to do one thing every time I leave my room to make it neater/cleaner/less cluttered. So something gets thrown out or put back in its proper place or in the donation bin every time I go to get a glass of water or go to the bathroom or whatever. Only a few days in, but I'm pretty impressed at the progress so far - two trash bags and one full box of donations out the door. Hopefully, I can keep up the habit long enough to get the whole room under control.