I'm pondering getting a tattoo. In fact, there are only two reasons I haven't done so yet. The first is cultural: I'm effectively a yuppie, and the yuppie class in England tends to look down on tattoos. I personally don't hold this opinion, but would I find myself in awkward situations with my peers?
The second reason is metaphysical: I'm not sure what tattoo to get. I feel like, if I start getting needled, my tat(s) should have some sort of theme to them. For example, in Robin Hobb's "Liveship Traders" trilogy, one of the characters gets a tattoo each time he has had insanely good luck. In XMen 2, one of the characters gets a tattoo for every sin. My guiding philosophy is an obsession with reality, so my approach to tats should probably reflect this.
The problem I have is that, as a reality-obsessive, any conclusion I draw about reality would be purely provisional. In other words, it's possible that I'd change my stance later - in which case I'd have to get the tattoo erased. So how about this for an idea: I get a tattoo only when I'm confident enough about my stance that I am willing to take the risk of erasure? Then, when someone asks me how sure I really am about X, I can say "this sure" and point to the symbol of X tattooed on my skin.
This would have an additional psychological benefit. One of the biggest problems that I have is a tendency to revisit old intellectual stomping grounds long after I should have just let the subject drop. If I am truly confident enough to get a tattoo, that will serve as a constant reminder that, absent any major new discoveries, I've done my due diligence and can quit wasting my valuable time.
At this point in my life, I hold two positions that fit the bill perfectly. The first is atheism, for which I would tend to go for the symbol described here (because I'm a maths geek and I love it). The second is evolutionary biology. I'm not entirely sure what to use for this - possibly the Darwin fish?
As an aside, I'd note that any theists or creationists who feel they have a solid reason for belief in God should speak now or forever hold their peace. After I get these tats, Mr Evangelism Target will have left the building.