Overcoming the mentality that I "might" need something that is taking up space was a huge hurdle for me, but once I did, it was so much easier to start downsizing.
As for my quilting, to keep myself organized, once a week I spend an afternoon putting everything away where it belongs and cutting scraps. I don't like working on the same thing ever day, so I pick two or three projects to work on for the week, at least one of which has to be a UFO. My ultimate goal is to have all of my scraps in an ikea dresser I have, all of my stash and UFOs down to one bookshelf, and all of my other craft supplies combined in a second bookshelf.
As for general purpose decluttering, each month, I choose a room to declutter. I go thru every drawer and cupboard and get rid of anything I don't need/want/use. I don't stop until I have at least one good-sized box to take down to the thrift shop. At least once a year, I pull every single article of clothing I own out of the closets and drawers and purge. I get rid of anything I don't use, doesn't fit, or is stained and worn.
Slowly, but steadily, I have been working my way through the abundance of "stuff". Once in a while, I do find myself needing a particular item I got rid of, but not often enough to make me regret decluttering overall. It's slow going, but at the same time, I am amazed at how much I have gotten rid of that I just don't miss at all. That being said, there is no way I could have gotten rid of everything all at once without going into some kind of shock.