Updated Apr 25
Maintaining this level is fairly easy.
My initial impetus for minimalism (as a teenager) was housing-based pressure. This meant, living in a small space, and relocating numerous times (both short and long distances). This has continued for a long time as an adult, and I foresee it continuing.
My basic value system (again, developed at an early age) is towards experiences and freedom. Objects are simply tools for those real pursuits.
Maintaining can come from a few angles.
First, there is the simple point of thinking carefully before you buy a new object. Do I really need it? How is it going to impact my daily life (e.g. helping efficiency and time-saving)? How is it going to further my overall medium/long term goals?
Second, some purchases are replacements. New item comes in, and old item goes out.
Nearly all of my physical possessions have practical use, and actually get used on a relatively frequent basis. It can be a good idea to keep track of usage (e.g. have I used this item in the past year?) I avoid keeping “just in case” items.
I used to be burdened with paper (old bank statements, letters, articles, printed information, university manuals, etc, etc). Scanning years of backlogged paper is time-consuming and tedious, but it only needs to be done once. Now, a lot of that starts as digital, and never takes paper form. Also, new paper can be immediately scanned, and so never builds up. Some paper (calendars, lists, notes, articles, work or university stuff, etc) is only around for a short time period and then tossed.
While I love physical books, I only own a small number, which are all for scientific reference. Otherwise, I rely on a large quantity of PDF format books, plus borrowing from libraries.
Some large/heavy items in my daily life don’t even belong to me, due to renting furnished rooms for the last few years. I have usage without the burdens of ownership (i.e. moving to a new room is quick and easy).
The last time I owned furniture, it was cheap second-hand stuff, which I donated back when moving.
I view most of my possessions as being temporary, disposable, consumable, and replaceable. This is in my mind from the moment I contemplate buying something, so my investment (both financial and emotional) will be relatively low from the start.
Disposability relates to mobility. If I moved across town, I would take a single taxicab trip to carry all of my stuff. However, the next time I move to a new city, I plan to downsize to an amount that would work for a long-distance bus/train/plane trip. Upon arrival, I would buy a few replacement items, but those would also be ultimately disposable.
Maintenance of this lifestyle is based on having a low financial and emotional investment, both when contemplating a purchase, and also when downsizing.