The Urge to Purge!

I'm planning on having a bit of a clear out of the things I just don't wear anymore, or that no longer fit. Spring is definitely in the air if I'm getting the urge to purge. I'm hoping to have a swishing party in March to get rid of my unwanted things in an ethical and environmentally sound way. How do you decide what stays and what goes? I am terrible for hanging on to things which might come in handy one day. Also, how often do you do a wardrobe clear out? I tend to do one biannually. At the beginning of Autumn/Winter and again at the beginning of Spring/Summer.