I think most people suffer from this in the modern age.
I take the following approach and do it on a room by room basis.
I go into each room, and go through each cupboard, shelf, etc and things either stay if I am using it recently, or it gets put in one of the following boxes.
Box for bin, box for sell, box for charity, box for not sure. The box for not sure sits in another room for 2 months, then go back to it and the info in there needs to go into one of the other boxes.
It takes time and slowly reduces stuff. Only bad thing is dealing with the human race when selling things online, so now most things i just give to charity.