It depends on the venue you're going to. I would not suggest wearing flip-flops in a crowded concert venue, purely for protecting your feet from being crushed (not to mention the lack of arch support when you're standing for most of a show).

If it's an outdoor venue and you have lawn seats, I'd suggest a summer dress or skirt and top. Bring layers so you can cover up as the night cools off and PLENTY of sunblock.

If you're seeing it in a theater or indoor venue, you can go as casual or dressy as you feel comfortable. Really, anything goes for concert attire (including ripped jeans), but if it's something you don't get to do often, I'd get dressed up a bit.