Standard-sized office paper with printing on only one side could be collected for printing on the second side. The used paper could have a message printed over the original used side to promote the idea and also avoid confusion about which side is the new one.
Universities generate a lot of printed material on standard-size paper. Most of it is printed on only one side of the page. It is looked at one time or posted somewhere for a while and then discarded. If there could be containers (a simple box) near every place where paper like that is used, with a sign saying "recycle office paper here" It could be collected for printing on the second side. The paper could be put through a printer and "First Side" or "Used Side" could be printed over the used side so people could recognize the paper was reused and not be confused about which side was new. The phrase "Save the Forest" or "Protect the Forest" could also be printed on the first side. A stamp could also be designed and made for doing that instead of using the printer.
Since newspapers and books and people's notebooks are ususally printed or written in on both sides of the page, that paper is fully used before it is sent to a recycling center but office paper is usually sent there after only partial use. "Second Side" doubles the life of a very common resource. It requires a small effort by everyone and the printing and stamping over the first side of the page is still a simple operation. When people start to see pages with "First Side" and "Save the Forest" and maybe a logo printed over the used side, they will appreciate the effort and it will inspire them to be more careful about other kinds of waste.