I had LOTS and LOTS of running tattoo ideas, some very large and convoluted. In the end, this is what I went with - the elevation profile for the Morgan Hill Marathon, which was my first marathon. (I'm now working on preparing for my third.)
The whole experience of training for and then running my first marathon was a really huge deal for me (as you may know, since I posted about it on my blog), and I think there is a lot of metaphorical resonance in these hills and in marathoning in general - the grueling difficulty, the ups and downs, the importance of being mentally strong in addition to physically strong. Those miles took a lot out of me, but they gave me back something too - the knowledge that I COULD finish a marathon. (And I've even finished another one since then.)
This experience is something that will be forever inked in my brain, and now it's inked on my arm too, where I can see it every day, and when I have moments of self-doubt, moments when I think to myself, "I can't," I can look at it and remind myself that I HAVE.