1. Double up your hanging space by installing an extra clothing rod in the middle.
Yeah, yeah, you'll probably ask - but what about my long dresses? What I did was that I hung them folded on hangers. That top clothing rod in the picture? Most of those are dresses. :)
2. Use velvet or felt hangers.
They are super slim and therefore affords you more space for more clothes! Best of all, the material grips your clothing and keeps them from slipping onto the floor. I found mine at Make Room.
For heavier coats or dresses though, I use wooden hangers as the weight of the item is just too heavy for these thin hangers.
Thick skirts and bulky sweaters can go here so that you don't cramp up your closet. I found this one at SM.
4. Use the bottom of the closet by adding an extra storage unit.
Keep out of season clothes at the bottom by storing them in any kind of basket or bin. The black bin I am currently using is from Ikea.
I would love to hear about your own closet organization. :)