Small closet organization begins and ends with discipline. You need to take the time to empty the closets, review the closet items with a critical eye, organize your stored belongings into like groups, and then replace the items in an organized manner.
The purchase of a closet organizer system is helpful but not completely necessary. The first important step is to empty the closet and review the closet contents. It is impossible to organize a small closet that contains too much stuff.
I cannot emphasize that point strongly enough. As a general rule at least 10% of the stuff in every closet is entirely disposable. Do yourself a favor and get rid of the clutter. It is not easy to get rid of things if there is a slight chance that they may be of use in the next 5 years. However, you will be doing yourself an immense favor by going through a process of getting rid of clutter. Get rid of 10% or 1 out of every 10 items. Get rid of more if you can.
The next step in small closet organization is to focus the contents of your closet. Closets are best organized in terms of like items. If it is a linen closet then limit the contents to sheets and towels. Closets become disorganized when they contain a vast array of items. I once had a client store her tennis racket in the linen closet for years as a result of stashing it there once while cleaning up for a party. Keep your sports equipment in one place, your cleaning supplies in another place, etc. etc.
You will likely find that some of the contents of your small closet are best suited for other storage locations. Move these items to their new home. Not only should you narrow the focus of the contents to like items but you should also pay attention to the frequency with which you access the items. Keep items you use frequently in centrally located closets. Store infrequently used items in closets that are harder to access.