The usual failure is scope. Someone decides to declutter the house, starts in the spare room, finds a box of photographs, spends two hours on it, and stops. The spare room is now worse than it was, and nothing else has been touched. Pick one room and do not leave it until it is finished.
Clear a working surface first
Before sorting anything, empty one table or stretch of floor completely. You need somewhere to put things down while you make decisions, and if every surface is already full, each decision requires moving something else first. That friction is what stalls the job halfway through.
Sort into four groups, not three
Keep, donate and bin are the standard three, and they are the reason most sessions stall. The fourth group is the important one: things you cannot decide about yet. Put them in a single box, write the date on it, and carry on. You are not allowed to think about that box again until the room is done. Most people find the decisions become obvious once the room around them is clear, and a few things turn out to matter more than they expected.
Handle every item once
Pick a thing up, decide, put it in its group. Do not pick it up, consider it, and put it back where it was. That is the single habit that turns a two-hour job into a six-hour one, and it is harder to avoid than it sounds, particularly with anything sentimental.
Do not buy storage yet
Buying boxes before the sorting is finished guarantees the wrong boxes. You cannot know what you need until you know what is staying. Finish the room, live with it for a fortnight, and only then work out what is still looking for a home. Most of the time it is less than you would have bought.
Deal with the undecided box
Once the room is done, open the dated box. Anything you have not thought about since putting it in there is a candidate for going. Anything you went looking for during the fortnight clearly belongs in the room, and now you know where. Do not let the box become permanent storage with a label on it, which is what happens if there is no date and no deadline.
The bit that keeps it clear
A room stays clear when putting something away is easier than putting it down. That usually means fewer things in the room and storage at the point of use, not a tidier arrangement of the same volume. If you find yourself tidying the same surface every evening, the storage is in the wrong place rather than the household being untidy.