One thing that has helped me is to go through things somewhat slowly, several times, getting rid of a little more each time.
Example: I started with "five" boxes of my son's baby stuff (in quotes, because I don't actually remember how many there were). There were blankets and quilts my mom made, different outfits, some toys, etc. I decided to drop to four boxes + one small box to start. I donated the other items from the fifth box.
Then, after a few months, I went through it again and got rid of more. Then, when I found out my younger brother was having a kid, I went through it again and pulled out stuff for him. Worked my way down to three boxes.
Then, after more time went by, I went through it again and donated more.
I now have two boxes, and I consider them to be as condensed as they'll get until either my son has a baby, or, if he chooses not to have children, they will go to my nieces/nephews when they have kids. The items I kept were his coming home outfit, his "was supposed to come home in" outfit (he was a preemie, so too small for the original outfit), a little suit, and the items my mom made for him. I hope those can be passed down, they are in great shape, and it would be nice if a baby had something that great-gramma made. If DS does have a child, I plan on cutting the remaining clothes and making a new baby quilt from them. I would hope that that preemie outfit wouldn't fit his baby!
Now that I quilt, I kind of wish I had kept more baby/kid clothes, but I can always make stuff from his grown-up clothes instead.