The easiest way for me to do this is to have a folder on a bookshelf set aside for completed work. When my children complete anything throughout the year it goes into their folder. Then at the end of the year I make piles of subjects and choose from there.
Some parents like to make fancy scrapbooks or notebooks to feature their children's work. If this is something they like to do, then they should go for it. For the rest of us, having a place to keep records that can be sorted later is usually the best option to keep our sanity.
Here is my post on Home School Records and all I have to say about this subject, but remember to relax!