Sometimes after finishing all the working flow, you just back-up, delete then for the computer and never ever look to then again. That's a really nice way to loose lots of chances of having new ideas about old works.
Of course, there are works with are very commercial or very simple and there is nothing new to be find there. But there are other with takes a lot of time for us to understand what we've got..
So, give a try to those old files..you my find something new =D