This Hamlet tattoo was submitted by Joh.
This above all: to thine own self be true,
And it must follow, as the night the day,
Thou canst not then be false to any man.
- Hamlet, Act I, sc. iii
Joh, however, prefers another quote:
I have to say, I love Hamlet, I love Shakespeare in general, and I love the message of the whole speech.
Plus, the excellent Clueless reference cannot be ignored:
Heather: It's just like Hamlet said, "To thine own self be true."
Cher: Hamlet didn't say that.
Heather: I think I remember Hamlet accurately.
Cher: Well, I remember Mel Gibson accurately, and he didn't say that. That Polonius guy did.