Almost everybody approaches having too much from the wrong end. They remove things, feel relief, and then slowly refill the space over a year, which means the clear-out has to be repeated. The volume in a house is set far more by what arrives than by how often it is thinned out.
So the useful work is upstream, and it is mostly a handful of small habits that cost nothing and are deeply unglamorous.
Put a gap between wanting and buying
Anything non-urgent goes on a list with the date you added it. Look at the list a fortnight later. A surprising share of entries have stopped being interesting, and the ones still there are the purchases worth making. This is not self-denial, it just removes the part of buying that happens in the first ten minutes of wanting something.
For anything over a certain price, make the gap longer. The number itself matters less than having one, because the point is to notice the decision rather than to make it hard.
Count the things that need a thing
An item that requires its own storage, its own accessories or its own maintenance costs more room than its own footprint. A bread maker needs a cupboard, a shelf of flour and the attention to keep it in use. A bicycle needs somewhere it can be got in and out without moving two other objects. Judging a purchase on volume alone understates the permanent ones considerably.
Refuse the free things
Most households take in a steady trickle they never chose: conference bags, promotional mugs, hand-me-downs accepted to be polite, a relative's spare crockery. None of it was decided on, and it is often the hardest to get rid of because nothing was spent on it.
Saying no at the point of offer is far easier than storing something for three years and then feeling guilty about it. Taking a thing to be kind and then housing it indefinitely is not actually kind to anyone.
One in, one out, but only where it bites
Applied across a whole house the rule is tiresome and gets dropped. Applied to the two or three categories that genuinely creep in your home, it works and costs nothing to remember. For most people that is clothing, kitchen gadgets, children's toys or books. Pick the categories that are actually a problem and ignore the rest.
Buy the thing that lasts, when you can
Replacing something three times means three purchases, three disposals and three periods of mild annoyance. This is the single argument for spending more, and it only holds for things in constant use. For an object you will handle twice a year, the cheap one is the correct answer and spending more is just a nicer object sitting still.
Expect it to be slow
Changing the inflow shows no result for months, which is why it rarely gets credited and the dramatic weekend clear-out gets all the attention. The test is whether next year's clear-out is smaller than this year's. If it is, the habits are doing their job, even though nothing felt like progress at the time.