I love a clear out and do them seasonally, or essentially any time one of those charity bags comes in the door or we are taking anything else at all to a charity shop. I do a capsule wardrobe now and am much happier so find it quite easy get rid of things.
As a general rule if I've not worn it that season or that year (unless it's something genuinely useful such as a pair of wellies or a proper waterproof coat) then it goes.
If I kept it from the last declutter and still didn't reach for it, it goes.
If I try it on and dislike any thing about the fit at all or it doesn't feel right 'somehow' or doesn't feel comfortable, it goes.
If it sticks out like a sore thumb and doesn't go with anything else I own (unless it's for a specific occasion) then it goes.
If it doesn't fit, or fit the way I want anymore; it goes.
If it no longer fits with my lifestyle (eg I no longer need to wear heels and am not in need of anything corporate or very formal for work or socialising), it goes. Being very honest with yourself about your lifestyle helps with this, and being honest about whether your things reflect you as you currently are or whether you bought them for a person you'd like to be- but aren't, which I used to be guilty of.
Anything that can be relegated to the loungewear/ugly house clothes/painting drawer if it's comfy (eg cardigans, old t-shirts, jumpers), then I do that to give it a second life until its done with.
If I have a few of a similar thing (eg 3 red jumpers) then I'll get rid of whichever I wear least.
I cant say I've ever missed a single thing I've got rid of.