Wearing clothes with slogans written on them is a bad idea socially. It is quite unlikely that anyone will ask you about it, and even less likely that such an interaction will result in any good. All the negative social effects are likely to overshadow the few positive encounters you may have. Even if you wear the clothes with the slogan in the appropriate social context, like a Less Wrong meetup, they don't add any value.
If you wanted to talk to someone about rationality, what do you think would help more in impressing them: a rationalist wearing normal, stylish clothes, or a rationalist wearing a t shirt with a slogan printed on it?
It is difficult to explain why clothes (or vehicles) with slogans printed on them are bad for you. If it helps, consider that the vast majority of people you see outside are not wearing clothes with witticisms, they have all decided that it is not beneficial. The exceptions are mainly religious, political, and other extremists.
Maybe mugs would work better than t-shirts for some environments.
What are your best short witticisms, suitable for use on a t-shirt, bumper sticker, or similar location? Ideally something that might make someone reading it think, or get curious enough to ask about it. Simple in-group identification is fine too, though.
For context, therufs is spending today at the NC Maker Faire making t-shirts, and asked me for suggestions this morning. As I was still mostly asleep, I wasn't very helpful.