For most people, no matter how big their closet is, it just never seems to be big enough to store all their clothes, shoes, and countless accessories comfortably. Yet usually, the problem is not really space, but effective use of space. So many closets have completely unused vertical space, and limited surface areas to try to squeeze everything on, and so it becomes almost impossible to keep the area in any sort of organized condition.
The solution is to use a closet organizer system to create storage-specific places where all the knickknacks that accumulate in closets to go. Many closets, especially in older homes, were not designed with optimal use of space in mind, especially as bulky bedroom furniture was expected to be able to store anything that couldn't fit into the closet. Today, however, the trend is to use some sort of multi-shelving system to get everything you wear into one place in your home.
Closet organizers can be as simple and cheap or as aesthetically pleasing and sturdy as you like. Modular systems like the Container Store's have standard wall-mounted strips that you attach to your closet walls, and then you can choose from a variety of drawers, shelves, and other storage options to take advantage of the space you have. While not extremely portable, systems like these can at least be taken down, and re-used in another closet.
You can also buy shelving at hardware stores if you want to save some money, but it will be more difficult to stay organized if you're just trying to set stacks of clothes on a flat shelf. Drawers will work better, preferably with different drawers for every type of accessory you want to store. Hooks are also extremely cheap and can make a huge difference in keeping clothes off of the floor. Many people find it tiresome to hang up clothes they will re-wear, like jackets, all the time, but wall-mounted and over-the-door hooks are an effortless solution to stay organized.