I wear whatever. Past 3 concerts I went to I wore jeans and a tshirt. Those 3 shows were all the same band, Dashboard Confessional (I know, I know, I'm obsessed). 2 were solo shows in NYC and the third was a big show outside of Boston. That show.... Man... Lots of 15 year olds in trucker hats. I'd shoot myself before ever joining that trend.
I wear what's comfortable. I sort of think it's lame wearing the band's tshirt during the show. I did buy 2 and my friends make fun of me for wearing them. Then again they make fun of me in general anyway