First tattoo!! ♥
The cresent moon for me stands for dreams and I've been drawing it on my hand on and off for the past 2 years or so to serve as a reminder to do a "reality check" to see if you're awake or dreaming. This habit continues on into your dreams and after a while you start asking yourself while dreaming, which then allows you to recognize that you're dreaming and become "lucid" and influence your dreams.
This method is called Dream Induced Lucid Dreaming (D.I.L.D.). There is also W.I.L.D (wake induced) which I experience as well and I've written about my first one here: http://jinx-in-the-sky.blogspot.com/2010/12/wild.html