I used to use a paperclip to do this but recently I received some colorful binder clips as a gift so I am using those. It has worked out perfectly, and that is the hack part.
Binder clips are good bookmarks, just flip up one of the side clips. The side clips don't get in the way of writing because you can flip the sides up.
Binder clips can hold extra information, especially if you don't have much pocket space, and this is what got me thinking about all the great uses of using these clips. Over the weekend I got a letter from my doctor, rescheduling an appointment. I couldn't process it in my planner at the time so I clipped it to my current spread so I wouldn't lose it. It worked perfectly!
(I have heard that some have even used binder clips to hold their pens when a pen loop wasn't available)