I have a box for each and keep a semester's worth and then toss it and start again. Anything that they are especially proud of is kept in a different box and at the end of the year we go through it together and decide what is kept. Most of DD's stuff last year got tossed when she brought home her portfolio from school. Any special projects like father's day, mother's day etc are kept forever.

I got caught last year when I threw out a few things and DD asked for them to show somebody. She got so upset when I said I'd thrown it away.