I just finished clearing my wardrobe this weekend and found 4 bin bags worth of stuff to donate, 2 bags that needed to be thrown out and an under bed storage box worth of out of season clothes. Even with all of those items leaving my wardrobe it is still fairly full, but I know that everything in there is something I will and do wear regularly.
The key I found was washing any dirty laundry so I have all of my clothes there for the sort out and then piling the clean laundry on the floor along with everything from the wardrobe. I gave the wardrobe a quick dust and assessed the storage I have.
My problem was that I had so much stuff hung up, I could never see it all so tended to wear the same few things again and again and was often too lazy to put everything on hangers so it just piled up on the bottom of the wardrobe so I introduced two sets of hanging shelves. I designated one set for tops and one for bottoms and then assigned shelf space to each 'type' - so the tops shelves were split into vest tops/short sleeved t shirts/long sleeved t shirts/lightweight jumpers/tunic tops and bottoms into smart trousers/work trousers/lounge pants/joggers/leggings/jeans
Then I went through everything with a donate bin and a bin bag next to me - anything damaged/with holes/zips that stick/missing buttons/threadbare etc went straight in the bin bag as often the reason the item hadn't been fixed was because I had 20 others or it was beyond fixing but I wasn't ready to part with it because it "just" needed an easy fix. Anything that didn't fit went straight into the donate bag as there's rarely any point keeping clothes I "might" one day need again. Anything that hadn't been worn in the last 2 years was donated and everything else I folded and assigned to the right category. Any category with too many items for the shelf space I went through and chose my top few items (and/or pared down when I had multiple almost identical items to just one or two) that I could not live without and donated the rest.
Any formal dresses had to pass the test of "If I was going to an event next week, would I buy this over any other new outfit?" If yes, I put it in a garment bag and stored it in my wardrobe if not it went in the donation bag.
I allowed myself one storage box for seasonal tops (I work in retail and wear specific themed and charity items that are "uniform" items only at certain times of the year e.g. Christmas jumpers) and holiday clothes that couldn't be part of my regular attire (eg ski pants/beach wear) but I only let myself keep items I used on the last appropriate holiday (if I didn't take those shorts on my last beach holiday I clearly didn't want to wear them so why let them take up room at home? If I didnt wear those ski pants when I went on my last ski trip, I'm probably not going to take them on the next one so why keep them?)