I use whatever is around… a receipt, a square of TP, a cat’s whisker once. I have a bunch of bookmarks but they’re never around when I need them (and I always forget to grab one when I grab a book off the bookshelf).

I use the receipt I get from the library when I check the book out. No danger of forgetting due date.

A sticker of cute dogs in space suits and the caption “we want fully automated intergalactic luxury gay space communism” because it reminds me of Ian M. Banks - also gets a laugh out of people 😁