After accumulating a lot of business cards, it's easy to forget exactly why they were given to you or how you were supposed to follow up. You could write notes on the back, but a pen-less option is this corner-folding hack.
Professional speaker Tod Maffin assigns each corner of a business card to specific reasons people want him to follow up with them about: sending a copy of slides, working with his company, hiring him as a speaker, or other. When he gets a business card, he surreptitiously folds the appropriate corner to remind himself how to follow up.
For urgent requests, he even has another level: the folded corner goes towards the front of the card rather than the default back.
You could adapt this clever hack to your particular followup reasons, naturally.
My Secret Business Card Corner-Fold Hack [Tod Maffin]