In a small flat every storage decision costs floor, and floor is the thing you actually live on. So the rule is simple: new storage should take no new floor, or it should replace something that already had some.
Up: the wall above the door
There is a band of wall in every room between about 180 cm and the ceiling that nobody uses, because nothing is ever placed at eye level there. It is the right place for the things you need four times a year — suitcases, spare bedding, the box of documents.
Put a closed container up there, not an open shelf. Open shelving at that height reads as clutter because you see it from below.
Under: the bed is a room you already paid for
A standard double bed sits over roughly 2.8 square metres of completely wasted volume. Two or three lidded bins on that space will hold an entire season of clothes. Lidded matters — under-bed storage collects dust faster than anywhere else in a flat.
Double: furniture that stores
An ottoman bench at the end of the bed is a seat and a blanket box. A storage bench by the door is a place to put your shoes on and a place to put your shoes. This is the only category where you add storage and lose nothing, so it is worth spending more here than anywhere else on the list.
What not to do
Do not buy small containers. Ten small boxes hold less than three big ones and take four times as long to search. In a small flat the container should be as large as the space allows, and there should be as few of them as you can manage.