Flickr photo by eagle1effiMark your calendars!
Really ... mark the calendar. That's the tip. But don't tell him, it's the element of surprise that's the romantic part.
Whether it's a cute note, a date night scheduled in, or a lipstick kiss, as he turns the page to write in his tee-off time for next month, he'll feel all warm and fuzzy inside when he sees your inscription.
Do you ever write sweet notes in places that you know your SO will find?