The most important thing to remember is that the music is not paramount; you're actions and appearance are what matter most.
Goddamnit, this is important, so listen up. Floppy hair is best, and don't be afraid to use some light product, though take care to make it look a bit disheveled. Any and all hair should be combed in a consistent direction, with best results being the sideways style. If you must wear something on your head, a fedora or a 70's era tuque is acceptable, as is a hoodie (only worn with the hood up while inside).
Facial hair is acceptable and often encouraged. NEVER wear a goatee, but feel free to sport a beard, the more length, the better. If you must, an ironic moustache is passable, but never for back-to-back concerts. If you're unable to grow a beard, at least have the decency to have some sideburns, preferably flared at the bottom. There's a reason Dylan from 90210 is epic. Also acceptable: mutton chops.
If you wear glasses, you have one choice and one choice only: Rivers Cuomo
T-shirts are OK as long as they are promoting an obscure band, an indie record label, or a solid and uncommon color. However, this time of year calls for something long sleeved, even with the show being indoors. Tweed sportcoats and cardigans are the go-to getup. Bonus points if either have some sort of elbow patch.
If your pants aren't an indigo color, you might as well stay home. Levi's or Old Navy jeans are for Foo Fighters fans. It's enough to have you tried for war crimes, so don't even think about it. Sizing is just as important as color. Find out your "mainstream" size and then go with at least two sizes smaller.
Footwear lends you two non-negotiable choices. The first is the timeless indie classic, the low-cut Converse All-Stars. Your only other option is a lip-on dress shoe. Again, this is non-negotiable. Tennis shoes are for Pearl Jam fans.
Now that you're fit to go outside and be seen by other indie fans, you better learn how to act. Yeah, the music may be great, but knowing how to act is the key to avoid discriminating looks.
If the show is in an intimate setting, there is no standing. You shall stay seated and view the band with a discriminating eye. Feel free to eat some food, maybe even pack that in your napsack. By all means indulge in a beverage as long as it's a microbrew from at least two states away from your location, or an obscure European brew.
If your location demands that you stand, for the love of God, please DON'T DANCE. Try and find a wall that you can lean on. Once you're good and posted up, you can tap your foot to the bass beat of the drum and/or gently nod your head. Don't forget to give a simple and conservative applause after each song. Any more than that and you might as well get tickets to a Rolling Stones concert.
Indie events are a family friendly affair. Bring your kids and be sure to bring a large bag of toys. Even newborns are perfectly acceptable to bring along. Just be sure to bring ear protection for little ones under the age of 1. Don't worry if your rugrats wander during the show, your fellow indies will keep them entertained, all the while viewing you as "the cool parent they never had".
When it's all over, buy a sticker from one of the opening bands (not the headliner) and converse with others. Be positive, but keep it in check. You know that sound guy had the rhythm guitar WAY too loud.