Do you remember those things that were supposed to save you space in your closet? They were a plastic rack with a hook on each end and you put your hangers through the holes in the middle. They worked, but they were pricey.
Sooooooooo....how about a cheap alternative? Well, I take the plastic rings that your drink cans come in, hang one ring over the hanger or thing on the back of my door and then insert the hangers through the holes. More than one hanger can fit in a hole and these things are amazingly strong. I've had the same one on the bathroom door for a year without a single tear! You can color organize your closet or group entire outfits together with them.