I would think that matching tattoos of something that you both identify with, perhaps some special event, or a special date. An example might be if you were to have had a date and it rained, a few raindrops might be appropriate. Perhaps you realized that you loved each other when laughing at some cartoon character. Maybe it was a small cafe that spent the night talking in. Or you go and see a certain event everytime it comes to town. The key is something the two of you identify with.