I do a cull at least three times a year. I also have only one and a half rails, 2 tiny shelves and one drawer, so I have to be utterly ruthless.
On the half rail I put things that are not appropriate for the time of year (big jumpers and tweed skirts etc in summer and sundresses in winter) but that I still want. I also put things I'm not sure if I want or not. If I'm still not sure after two culls maximum, they go.
Jeans and jumpers live on the 2 shelves. If the two shelves are full and I want to buy jeans or a jumper, something has to go from the shelf to facilitate a purchase.
In the drawer I keep pyjamas, thermals, swimwear and any t-shirts/vests that a) I only wear on summer holidays or b) that are for layering in winter.
So that leaves the main rail for normal day to day stuff. This is divided into sections for jackets, dresses, skirts, tops. If tops get a bit shapeless they first of all get relegated to the winter layering drawer. If they don't then prove their worth in there, out they go.
If anything is too big, too small, or not good enough quality - it goes. I don't keep anything in the hope it might fit one day. If it doesn't fit at cull time - it's gone. Tbh though, my size (although maybe not shape) doesn't change much and hasn't in 6 or 7 years.
I don't have many clothes, but I do like to try to make sure that anything I own at any point is earning its place. That way, my few clothes are more use to me than lots of clothes that are not quite right.