This is necessary.

as a member of the audience, taken low and against my chest so as not to disturb others

As posted last week, I attended a concert a number of months ago (that I just now got around to talking about) where some fancy nouveau riche family in their gaudy clothes and balcony seats let their little brat children talk through the entire performance. It may not be a special occasion for you, but some of us come to the symphony to enjoy it. It is a special occasion. Speaking 100% realistically, every concert, no matter how bland or awful or plain, is an event that will never again in your lifetime happen again. Even if performances are scheduled back to back, minute differences in not only the performers and their moods and the conductor and his, the temperature and lighting and the weather, but also the onlookers, no one show will ever be repeated, for better or worse, and when I go, I want to be able to enjoy it.
If I wanted to hear you crinkling with something in your purse or chatting with someone, I’d go sit on a bus or in a cafe. I paid money for a ticket, and I want to sit in respectful quiet and enjoy what the ensemble or performer(s) have worked to prepare and perform.
For you first-timers, let’s do a quick run down of what you should know and do for your first symphony visit.

1. Be early- Have plenty of time to get there, find parking, find the entrance, whatever else. You may also be surprised to find a talk before the performance introducing the piece(s) being performed, or just enjoy a walk around the hall.
2. Dress respectfully- Unless it’s a special occasion, the symphony is generally not a black-tie affair. Wear what you might wear to a nice-ish restaurant, no tie necessary, but certainly not out of the question if you want to dress up. I wear (nice, dark) jeans with a sweater or jacket to the symphony, but that’s about as casual as I’ll get. No shorts.
3. Find your seats early- That doesn’t mean you have to sit there for twenty minutes before the program starts. Find them, and then get up and walk around or stand outside for a while before it begins. You’ll be sitting for a couple of hours anyway. Get (back) to your seat and settle in with enough time that you’re not scrambling over everyone who’s already there and can get adjusted, place your bag down, take off a coat, and turn your mobile phones off. 
4. Yes, off… I at least turn mine to airplane mode. I have alarms and reminders for things all throughout the week, and I don’t want any of them to go off during the performance. I’m also usually kind of low on battery life by the end of the day, and I won’t be using it during the show anyhow. Truth be told, most concert halls I’ve been in get very poor if any reception, which is nice, but some people will still get service. Turning it off and putting it away will keep you from pulling it out and checking it out of habit.
5. Dont’ clap until the piece is entirely finished- Maybe this one is a bit difficult if you don’t know when exactly a piece is finished, but if in doubt, wait. Once the other thousand people in the audience start clapping, it’s a pretty good cue the piece is done. So that’s an easy one. Give it an extra 5-10 seconds before you start clapping to make sure you should be. Everyone will appreciate it.
6. There’s generally an intermission, and people will storm the bathrooms. It may be worth it to go, and the intermission is usually 15-20 minutes. There may not be such crazy lines at the toilets after a few minutes have passed, but be sure to get back to your seat early enough for the program to begin with you there.
7. There’s lots (and lots) of clapping at the end of a concert, especially if there’s a soloist, or it’s a premiere or something like that. A soloist will leave and return and leave and return and perform an encore, and leave and return and on and on. You can leave at any time during this clapping, but be swift and discreet about it. It isn’t (necessarily) rude, as some people obviously need to get home, but if you get stuck in another encore, stay. It won’t last long.

Honestly, those are the basics, and it can be summarized even more. The list of things you should or can do at a concert is shorter than the list of what you can’t, obviously, and in my opinion, it would look something like this, in no particular order:

• sit quietly (silently, even) and listen
• clap after the piece is finished
• look through your program quietly without dropping it
• hold a loved one’s hand if you feel so inclined

If you must cough or sneeze or make a noise that isn’t part of the piece being performed, at least wait to do it when there’s a lot of music happening, and not during a flute solo, or the end of Mahler’s ninth. 

Kids? Well, I’m allergic, but I’m impressed to see parents bring kids to the symphony with the intent to culturify or educate them, and I’m even more impressed when those kids are (generally) well behaved. So that I don’t mind. I once tapped a lady on the shoulder who was sitting in front of me because she kept wanting to teach her child about Dvorak’s ninth while Dvorak’s ninth was happening. She immediately shut it, and after the performance, she even apologized. I felt band and told her I was impressed with how well-behaved and interested her child was and that it was rare and to be commended. I even felt bad. If your child is a hellion who can’t even behave in a loud restaurant, what makes you think he’s going to sit quietly in a dark room for two hours while people play boring music? Maybe get a babysitter.

Don’t drop your stuff. 

While it might be part of an offstage section of instruments in a Mahler score I’m not familiar with, I generally don’t think that the sound of programs hitting the floor is a pleasurable addition to my symphony experience. What makes it worse, they’re often made of that slippery glossy paper. So don’t set them teetering on a silken knee or your pant leg. They’ll fall. It’s often too dark to read a program anyway, unless you have one of those lights, which I find at once classily opera-house-y and rather annoying. Put your program under your leg or in a bag, and put that bag down somewhere or hold it so you won’t dump your stuffs out all over the concrete floor during the pizzicato movement of Tchaikovsky’s fourth.
Something else to remember is that a concert hall is designed so that things are clearly heard. Usually those things come from the stage, and reverberate in such a way (if the hall is designed well) so that they fill the auditorium and you hear the music around you, with very little echo. So you may think that whispering to your neighbor about how much you love spaghetti isn’t such a big deal, but for those of us listening intently, it is. You can be heard. There is very very little that must be said during a performance, if anything, so if there is, say it early or wait.
I’ve wanted to draw my companion’s attention before to a certain part in the music, and with a little preparation, this can be done easily and without disturbing others. Whatever it is you want to show, share it before the concert. When the time comes during the performance, all it takes is a nudge, a tap, and a head nod toward the stage for the person to know that the hammer crash of Mahler’s six is (or should be) coming up.
Also… as tempting as it might be, don’t take pictures, and for goodness sake don’t try to record the program. Now, this is something I’m guilty of, but I must say, I feel confident enough that my sneaky photo-taking skills didn’t disturb anyone. No flash, no bright screen to light up the audience. If you weren’t looking at me, you’d never know, but even then, I know it’s a no-no. I should resist. But like at Gurre-Lieder the week before last, it’s just… so awesome (literally awesome) to see that kind of ensemble on stage… that I had to. Anyway, don’t do it. If you do, turn off your flash and cover your screen so no one sees that you’re doing it.
And that’s really it. I had an Australian jackass sit next to me for part one of Gurre-Lieder who insisted on shticking his elbows and knee into my chair and sprawling out as wide as he could and had the audacity to speak (not whisper, but speak) to me during the performance and say “Would you mind not putting your elbow on my side?” He was clearly in a huffy mood because he complained about everything, refused to hold his wife’s hand and was very eager to leave when it was done. Just don’t be a jackass.
Do everything you can to keep the peace: be early (or at least on time), stay quiet, still, enjoy the program, listen to the ushers, clap when everyone else claps, and you will have a great time, or at least you won’t piss off everyone around you.
Thank you for your attention. Please enjoy the show.