I've thought plenty of times about tattoos/piercings but I have none myself. Just haven't bothered yet.
I've thought about the piercings I'd want to get, and that's two silver rings in the upper cartilage of my left ear (my first girlfriend wanted me to get those, that was about four years ago now) and a silver stud in the earlobe (the only piercing my dad ever got).
As for tattoos, my second girlfriend wanted to get a paired tattoo of Jack Skellington on my left shoulder/upper arm and Sally on her right so they'd be standing together when we were. I thought that was sort of a cool idea. She turned out to be crazy and I left her. Go figure.
Aside from that one, I've got nothing I'd really want that means anything in particular. I've always wanted a tattoo of Mjollnir somewhere, and/or Gungnir, or possibly matching ravens on either side of my body (upper arms/shoulders perhaps, representing Huginn and Muninn - actually I've just had a thought on this). I'm not hugely educated on it nor do I claim to be, but I'm a fan of Norse mythology.