Tattoos, in addition to decoration and social signaling, can also serve as reminders: something you stand for, a phrase that inspires you, your girlfriends name. Yarmulkes also remind - that G-d is above the wearer. The big difference - apart from the content - is that yarmulkes fall off while tattoos don't. +1 for Tattoos! This seems like it would make the yarmulke the inferior of the two reminder tools. But turns out that habit is the ultimate reminder - not the content alone. Keeping a yarmulke on your head is a habit - it requires active engagement to keep it on.