Hang a tag on each item. If you don't wear that item between now and that date at least once, I can safely say you won't wear it at all within the next year. Translation: that item no longer serves you and it is time to let it go. If you do wear it now that you can see it (which is sometimes half the battle), once or even twice, and feel good wearing it, then the item stays.
When you remove the clutter, you can start to see what you have. You'd be amazed at how many outfits you can make with fewer items hanging in your closet.
After I help my clients remove clutter from their closets, they tend to look at the donation pile (mountain) and then back at me with fear in their eyes. They're seeing only what they're getting rid of, not what they are gaining. This is the perfect time for my favorite part of the entire process, when I go back into their streamlined closet and create 20 new looks they never saw before using less clothing than they originally had. These outfits were there all the time; they just couldn't see them.
Now, imagine the pieces hiding in your closet. It just takes asking one question to find out . . .