Being a college age female, I find that most of my clothes are hanging up in my closet. This was not easy to do in a dormstyle of living however. Being the neat freak that I am, I have my tops color coordinated, beginning with white and ending with black. I then have my jeans hanging from light to dark, followed by dresses and jackets. I have very little closet space in my sorority house. I found the best way to utilize this space is by purchasing ultra thin velvet hangers. The hangers maximize your closet space by shrinking the space that hangers would normally take up. So your closet are more compact. Here is what to look for at the store.

These sell for about $10 for a package of ten. They are made out of velvet to prevent clothes from falling off the hanger.
Another way to help utilize closet space is to put in a shelving unit. While you probably cant drill holes into your closet at school, there are other ways to go about doing this. They make stand up shelving units that can be placed on the floor of your closet. You can fold any type of clothing and stack it inside the cubes. They make several different kinds as well. They have sold plastic singular cubes which can be stacked, or wire cubes that are already attached to one another. You will have to choose which works best for you.
Here is one example of what to look for.

Another great way to utilize your closet space is to purchase a rack to hang over the side of your closet door, preferably on the outside as to not take up unnecessary space within the closet. I have one, and it is a life saver. I hang my purse, sweatshirts, scarfs, umbrellas, hats, and anything else you could think of. These are extremely cost efficient as well and can be found at walmart for under $20.
Here is an example of one may look like.