Here are the necessities for a really packed, impossibly hot and sweaty and moshy type of a concert:
1: Don't accessorize.
That's right, I said it. I once made the mistake of bringing a purse and a faux fur stole to a Modest Mouse concert, and I really wish I hadn't-- it's just an extra thing to carry around. Jewelry is okay, but only if it's very hearty, because you might lose it.
2: Wear tough-girl shoes.
I'm sure you can guess the specific kind I'm thinking of. Unless, of course, you want black and blue toenails and for one of your sandals to join the random shoes littered on the floor after the show.
3: Wear something cool.
Even if it's snowing outside, just think of how you'll feel once you're packed in a tiny subterranean room with 100000 sweaty bodies falling all over you.
4: Leave your hair down.
You know that fantastic post-concert hair, where it gets all wavy and disheveled, like you've been having sex on the beach? (at least, that's what I imagine it's like...) If your hair is up, it will end up all sorts of ridiculous, but leave it down and ready to be influenced, and you can start the next day with a Sharpie X on your hand, rings under your eyes, and a little bit of extra street cred.
5: Look really hip.
I know, I know, this goes against everything else I've said. But don't forget about the potential rock star encounter. *wink*
6: Wear earplugs.
Hearing stuff the next day is cool! Even if you look a little bit lame.
7: Bring an extra brassiere.
Or be willing to sacrifice the one you have on-- you'll want an undergarment to throw onstage, right?