I don't see any problem with a paperclip.

My solution is to use a sticky flag that protrudes from the page. When I no longer need it, I can easily remove it and keep it on the back inside page of my passport. When I'm really on top of things, I remember to put it on the page where I am stamped into Europe. The border agent will look for it when I leave. My passport is now 72 pages long and filled with stamps, so it is quite tedious when I forget to do this.