I can't keep everything. It's too much. But, everything that's in sketchbooks is kept. I have all of my sketchbooks, right down to the very first one. Old prints go in the garbage, some older digital pieces are saved with the rest of the really old crap, but most get deleted. I'll never use it again.
I want to make sure I don't hold on too much to the past and leave room to grow and create new stuff. I do keep some, I just can't keep everything. But, I kept everything when it comes to my comic series. Even the original script. I'll never use the old stuff, but it's documented history of this project.