I don't have a tattoo, even though I've given thought to what I'd want for years now. I knew I wanted to wait until I was an alumna before getting a Phi Mu tattoo, though, and I also maintain that I don't want to get my letters, despite my alumna status. There are still several ideas floating around in my head, but nothing totally solid yet, and I still don't even know where my tattoo would be located.
Idea #1: a quatrefoil (our badge shape, of course) with a cycloid (coincidentally ASA's badge shape; I'd use it because I'm a Steelers fan) inside it. Two passions, one tattoo.
Idea #2: a quatrefoil with three stars inside.
#3: Three stars.
#4: A hand holding a heart.
#5: A hand holding a heart, with three stars seeming to shine out from the heart. (This is the most likely option at this point.)
#6: The words "Love, Honor, Truth" in cursive.
...all of mine are taken from symbols of my Fraternity (or from our creed, as in #6), but they don't include the letters at all. Perhaps for Pi Phi you could choose just an arrow? Maybe a diamond (the basic shape of your coat-of-arms, of course), or a diamond with an arrow in it? Also, do a Google search and see if you find anything inspirational there from other Pi Phis. I would personally avoid getting a carnation (unless you found a VERY good line drawing of one that your artist could copy), because they often don't turn out clearly, or if you just say "carnation" to a tattoo artist, you may wind up with a rose or a chrysanthemum.