In my experience, cheat sheets typically consisted of everything one could enter on a 5" x 7" file card both sides. With this limitation, the most effective strategy would be to write down useful mnemonics rather than anything that could simply be copied verbatim. I allowed cheat sheets from time to time in some of my courses. I found that generally the process of creating a cheat sheet was a learning experience for many students in that it helped fixed things in their memories, with the result that there was often little actual need to refer to the sheet during an exam. However, the poor students created poor cheat sheets on which they tended to rely much more than the better students and thereby just perpetuated their mistakes and lack of understanding. For poor students it was a matter of "garbage in, garbage out".

Thank you Karl.