Instead of scribbling notes / to-do lists / shopping reminders etc onto a fresh piece of paper (be it a post-it-note, an A4 sheet or on new page in a notebook), use pieces of cut up card that would otherwise have gone straight into your recycling bin. Then recycle the card pieces once they've been used for note-writing. Any habit that reduces the amount of stuff that we need or want to buy / acquire is a step in the right direction.