When I'm out and I need to save a page in my book, I make a quick bookmark using a business card. I alter to business card with 3 well placed tears to keep it from sliding all over the place or falling out when the book is in my bag. Here are the steps.
Step 1: Fold Card in Half.
Hold the card and with the thin side facing you and fold the top flush with the bottom.
Step 2: Make 2 Tears.
Make two tears equal distance apart starting from the folded side going half way down.
Step 3: Push Up and Make the Last Tear.
Push up the center of the card and tear one side separating it from the card.
Step 4: Place the Card in the Page.
Place the card in the page you want to save with the card on the back of the page and the slot you tore out on the front. Make sure the card is snug. The extra effort put in to tearing out slots in the business card will ensure it doesn't slip and slide when your book is in your bag.
I hope you enjoyed my instructable.