(Photo NOT my own, courtesy of original blog post on robandjen.com)
Each year on January 1st, turn all your hangers around backwards. As you wear the item of clothing, hang it back up on a hanger facing the correct way. After 6 months, 12 months, whatever you like, you can easily tell which items you haven't worn. Then you need to get brave and donate them. Unless you have a really, really, like seriously good reason. I think I'm going to try this. I'll let you know how it works out!