Texace, the surest way to avoid getting a lecture is to simply not bring it up. Remember, their evangelical goal is to sell you on their worldview, so think of their shirts as being like Tommy Hilfiger or Nike clothing - just brand advertisement. No big deal - they can and should do it if they want to, and so can you, although wearing clothing with an atheistic message may only end up attracting those people who bother you in the first place.
In the case of unsolicited evangelism, I would suggest simply politely declining and then walking away or changing the subject. Most of these people might mean well, but many can get downright arrogant and patronizing, and this is annoying, yet amusing, considering the nature of religious faith. Attempting to put these people in their place via debate just breeds more hostility and you're not going to change any minds that way, as I've observed conversion (deconversion?) to atheism is a very personal thing that requires much deep thought.
If you're serious about your atheism, I'd suggest reading
good books about the subject (Smith, Martin, Nielsen, Flew, Drange, etc.) to achieve a better understanding. Also, just be glad that in America we still have that pesky Bill of Rights which prevents these people from
forcing you to believe as they do, or else.
