This is a subject near and dear to my heart. Having gone to, truthfully, hundreds and hundreds of concerts in all types of venues over the course of my many years (the first was the Beatles), I'd say there is just no way you can go wrong as long as you stick to fairly inconspicuous casual. You don't want to pick up on the performer's style unless you are a super-fan and really young. Otherwise, the goal is to get the rest of the audience to ignore you.
I am headed off to the U2 concerts in Montreal this weekend. Packing comfortable jeans and tees. I prefer to get close to the bands I go to see, as my eyes aren't all that sharp -- and, since it's live, I like to watch the people, not the screens. So, I prefer general admission, standing. When it's a superstar band like U2 playing, this usually means one has to stand in line for hours before admission and also stand throughout the show and while waiting to exit the venue. That calls for comfortable shoes, preferably ones that can be stepped (or pogo'd) upon without too much pain. It also helps to wear a platform or a thick heel to give you a little height advantage. While waiting outside, you do need to be prepared for the weather.
Now I realize that basically I am just seconding everything that Janet said! And the little cross-body bag is an idea I use, too, for the reasons Janet cites. Plus, it takes less time for security to search it, so you get into the venue faster.
