Rehang all of your clothes backwards (the opposite of the photo) - with the hanger positioned so that the end of it (the curved part that actually hangs on the closet rod) faces outward toward you as you look at it (this is easy to do, but hard to explain).
Now, when you take an item of clothing off of the hanger to wear and you are hanging it back up after laundering, put it back in the closet the usual way (with the curved hanger facing towards the back of the closet, as in the photo). Do this with each article of clothing that you actually wear.
After six months take a look at all of your clothes as they hang in the closet. You will easily be able to note which ones never got worn because they will still be hanging backwards. Take these out, put them in a bag and bless someone on FreeCycle (a free-for-all online recycling forum) or your favorite thrift shop.
Go a step further: While you are in the initial process of rehanging your clothing, take a few extra minutes to hang them according to color. Use colored plastic tubular hangers. Blue skirts, dresses and tops get hung on blue hangers. When I did this, I found that I had enough hangers in my closet already and did not have to buy extras. But if you do need to buy more, they are very inexpensive and can be found at dollar stores in a variety of colors.