For me, i'll cross out the page I am not using. So I always know which page is the actual page I am using.
also, if I am using a new paper with 2 sides blank, I will only use 1 of the sides. The other side is left empty. When the information is overdue, I can cross out the page and use the back later on.
hope this helps!
Mostly mine is printouts of drafts of things I have written. So I read through it and make corrections on the computer and then I have 40 pages spare printred on one side. I guess I could mark the printed sides with a stroke of the pen so I know they are used but usually I just put it in the pile with all the other half used pieces.