I used to save so much more than I do now. The truth is, as much as I love their artwork, I realized I wasn't really ever looking at it. It was just taking over our entire playroom! So now I'm pretty ruthless.
I keep a cardboard magazine holder for each girl in one of my guest room closets. If it's an important or unique piece of artwork, a special assignment or test, or anything else I might want to keep, I toss it in there. At the end of each quarter when report cards come home I pull out the holders and go through them. Everything gets organized and I make hard decisions about what I *really* think should stay. That pile goes into a folder so the holder is empty for the next quarter. I add to the folder each quarter, and at the end of the year I do one final look through to pare things down. Then I rubber band the folder, mark it with the school year, dates, and teacher's name, then file that away in a milk crate. I do allow the girls to keep some of the things they like that I've decided against, if they want, but they also have limits on what can be in their rooms...if their bins, drawers, etc. get full they're required to clean them out knowing they either make room or they can't keep any new projects.
My girls are in 1st and 4th grades, and everything I've saved fits into the two milk crates I have in the closet with room for at least another couple of years' worth. And I haven't yet regretted losing anything I've tossed, so the system is working so far.