Buying storage for a full closet usually makes it worse. The space problem is almost always a quantity problem, and it has a cheaper solution.

Find Out What Is Actually in There
Before reorganizing anything, establish what is being used. Turn every hanger to face one direction and reverse it when you wear the item. After a month, the division is clear and usually uncomfortable, because a small proportion of a wardrobe accounts for most of the wearing. That information is what makes the rest of the exercise possible. Reorganizing a closet where sixty percent of the contents are unworn is arranging a storage problem rather than solving it. The unworn items are the space, and dealing with them is the only intervention that produces lasting room.
Do the same for folded items by returning worn pieces to the top of each pile. After several weeks the bottom of each stack is the part of the wardrobe not participating, which tends to be a surprising collection of things bought with good intentions.
Resist the urge to act immediately on the results. A month of data is better than a weekend of judgment, because the weekend version keeps anything that feels like it should be worn rather than anything that is.
Diagnose the Unworn Items
There are only a few reasons a garment is not worn, and each has a different response. Fit is the most common and the most fixable. Trousers too long, a waistband that gapes, sleeves wrong, a shoulder that pulls. Most of these are inexpensive alterations that convert an unused item into a worn one. The second reason is that the item matches nothing. This is a combination problem, and occasionally one modest purchase brings several orphaned pieces into rotation. A neutral cardigan or shoes in the right color can rescue three or four items, which is a better use of money than more storage.
The third reason is that the item needs a repair. Pilled knitwear, a missing button, a hem coming down, shoes needing attention. These are not unwanted items, they are broken ones, and an afternoon dealing with the whole pile usually restores several garments.
The fourth reason is that you do not like it. That is a complete answer, and the correct response is to pass it on. Keeping it does not recover the money and it does occupy the space you are trying to create.
Use Vertical Space You Already Have
Most closets waste the space above the rail and below the hanging clothes. The gap under shorter garments like shirts and jackets is frequently a third of the total volume and holds nothing. A spare shelf, a sturdy box or a drawer unit you already own fills it. Doubling the rail is the other common gain. Where garments are short, a second rail hung from the first using any sturdy cord or a spare rail creates a complete additional row. This is the single largest capacity increase available in a standard closet and it costs nothing if you have anything to hang the second rail from.
The shelf above the rail usually holds items badly because they are stacked loosely. Using boxes you already have, even shoeboxes, turns that shelf into usable storage with visible contents rather than a pile that collapses.
Doors are the forgotten surface. Hooks on the inside of a door hold bags, belts and frequently worn items, removing them from the rail where they take disproportionate space.
Change How Things Are Folded and Hung
Knitwear should be folded rather than hung, because hanging stretches the shoulders permanently. This also frees rail space for the structured items that genuinely need it, which is a reallocation rather than a reduction. Filing rather than stacking transforms drawers. Folding items into rectangles and standing them upright, so each is visible from above, means the whole drawer is accessible and nothing is forgotten at the bottom. This is the single most effective change to drawer storage and it requires nothing but a different folding habit.
Hanging similar items together, by type and then by color, makes the contents legible. A closet where you can see what you own is a closet where more of it gets worn, which addresses the underlying problem rather than the symptom.
Remove dry cleaning bags and packaging, which take space and trap moisture. Matching hangers, if you have them, create noticeably more room than mixed ones because they space consistently, though this is a marginal gain compared with the others.
Rotate by Season
Storing out of season clothing elsewhere is the most effective way to make a small closet workable, and it requires only a container and somewhere to put it. Winter coats and heavy knitwear occupy a large share of volume and are useless for half the year. Under bed space, the top of a wardrobe, a suitcase that is otherwise empty, or a vacuum bag all work. The container matters less than the discipline of actually moving things twice a year, which also provides a natural moment to notice what went unworn through its own season.
An item that was not worn during the season it exists for is not going to be worn next year. The seasonal rotation is the best point to pass those on, while they still have value, rather than storing them for another twelve months.
Keep a short list of what is in storage. The common failure of seasonal rotation is forgetting what was packed and buying a replacement, which adds to the quantity problem the whole exercise was meant to solve.
