The blind prophet Tiresias is a frequently recurring figure in Greek mythology. He lived approximately seven times as long as most mortals, and served many of the rulers of Thebes, so it's not too surprising that he shows up pretty often. Even dying didn't totally end his career, since his shade gave advice to Odysseus during his visit to the Underworld.
Tiresias wasn't born blind, and many versions of his history say that he wasn't born with prophetic powers either. Some myths say that his sight was taken from him by the gods as a punishment for giving away their secrets, or for catching a glimpse of Athena bathing in the nude. The most interesting take on Tiresias, however, is that he came across two snakes mating and hit the female with his staff (why he did this isn't clear; some versions say that they attacked him upon seeing him, while others don't give a reason on Tiresias' part), which resulted in his turning into a woman. He (or, more accurately, she) kept this form for seven years, during which she served as a priestess of Hera, had children, and (according to some versions of the myth) also engaged in prostitution. When she came upon the same snakes, she struck the male, and turned back into a man. Due to his unique experience, Zeus and Hera asked him to settle their dispute as to which gender enjoys sex more, and Tiresias sided with Zeus in saying it was the female. This angered Hera, who took away Tiresias' sight, but Zeus tried to make up for it by granting him a long life and prophetic powers.
So I suppose the moral is, if you're unsatisfied with your current gender, be sure to hit any copulating snakes you might find. But try not to piss off Hera.