Rescue rather than reject.
I find it easiest to dump the whole toy bin out on the floor and then have my daughter put back her very favorite things. What's left on the floor is trash, broken and used up toys, things she forgot she had and a few favorites she overlooked. She sorts through the remaining pile and rescues the few things she can't live without. The rest goes to the trash or donation bin. It is so much easier to find favorite things and rescue them than it is to try and find something to get rid of. At first I sat with my daughter and coached her through this process. Now I can send her to do it on her own. In fact that is what she’s doing while I write this.