When Andie started school (9 years ago!) we bought 20 26"x12" flat files and started filling them up with mementos for each year.

Now each child has a box for each school year that contains all the good stuff we want to keep. Some have parts of halloween costumes (like the giant white sunglasses Jack wore with his Willy Wonka costume), some have party invitations, pictures of friends, tickets to plays, programs from performances.
Because all three of our children have gone to the same elementary school, it's especially fun to compare the school projects they've all done -- we now have a complete Furber set of the Old Lady Who Swallowed the Fly from the Rancho Romero Kindergarten curriculum.
Every once in a while I sit down with a stack of them and try to clean out the contents, thinking the kids can't possibly want all this when they move out and have to store it all themselves. But then I get lost in the memories brought back by each little item, and I keep it all.