When something comes into my house that I know I, or the kids, want to keep, I put it in one of two places.
If it is something that I know I want to include in their scrapbooks I put it immediately into a box-this box is easily accessible (right next to the kitchen), and on top of the armoire that holds my kids scrapbooks.
The box has six clear folders in it for each child.
When it is time to assemble my kid's scrapbooks for the year (I usually do this when I have a burst of energy at the end of the year), I get out each of their folders and decided what I am going to include in the book. (You can read about how I scrapbook here.)
If the keepsake, paper, artwork is too big-or not 'scrapbook-able', I have big plastic bins that I keep up in a closet in a bedroom. This is where I throw the fancy art work, special t-shirts or clothes, baby keepsakes, special little toys or books, school journals or notebooks, trophies, medals, postcards-I think a few of the boys might have a cast or two in there-lots of special mementos. I just bought really heavy duty bins for the attic since Abbey's and Isaac's and Matt's were full-I emptied out their closet bin and filled up their heavy duty attic bins.
I am careful about what I choose to keep. With older kids, they let me know what they want to put in their bins, but I try to keep in mind what they would love to see in 10, 20, 30 years.
(I love that I have a red superman cape hanging on my coat rack.)
I save special t-shirts in their bins over the years so that at high school graduation I can have a quilt like this made from Jane. She also makes quilts from baby clothes. Here is Isaac's-he chose which 12 shirts he wanted to include.