I do keep my lists electronically but I don't keep track of completed actions. I just don't have anything that requires that amount of supervision. I was going to say that this is because I am a student but I know you are too. I do know that those in a business environment might rely on completed tasks for reviews though.

Still trying to keep it simple, I suppose you could copy the task, complete it, then edit the copy. For those that duplicate actions on their to-do list and their project list could mark it on their project. But right now I have this in mind for micro-projects, though I don't think it is limited to that.