When I was younger I'd say something like, 'Sorry, busy having sex with my girlfriend'.
Then I realised that was just stupid. I usually get away with 'not interested', and sometimes bring out the Atheist card. Some of them are just so stubborn, though. Last time, I said 'sorry, got to leave the house in a minute because I have an exam', which was true, and they just didn't get it >_>
I mean, I have no problem with religion and I enjoy studying them, but I do not take kindly to people trying to shove it down my throat. And I have major problems with the beliefs of the JW branch of Christianity.
Should add that I'm an unlucky sod that lives about five minutes away from a Kingdom Hall...