This is inspired by two concerts my wife and I attended on the past two weekends (after not seeing any for a long time).
1) Act like you will have a say in whether people sit or stand.
We went to see Steve Miller at an outdoor amphitheater last Saturday night. Great concert, great jamming, marred only by the incongruous rapping by the keyboard player during their lengthy "Fly Like an Eagle" jam, and a small group of dopes right behind us.
We were on the lawn, and when the music started, the people in front of us (and many others) stand up and start dancing or otherwise rocking in their own way. So we stand up, only to have the group of (and I'm guestimating here) 45-70 year olds behind us start hollering for everyone to sit down. After a short time, I turn around and semi apologetically say "sorry, but if we did sit down, we wouldn't be able to see over the people standing in front of us."
They ignore this, and continue yelling. To make a long story short, they started throwing stuff, and hit the woman standing next to me. I turned around and told them to cut it out. I found it pretty irritating that I had to suggest to a group of people old enough to be my parents that I'd put a beating on them if they didn't stop acting like little brats.
At a rock concert, you just might have to stand up.
2) To follow on to the first rule, you (in this case, the artist) don't need to make the token black guy in a band rap.
3) If it's a mellow show, let the artist know you're still there.
We saw Shawn Colvin and John Hiatt play solo acoustic sets last night at the 9:30 Club. Both, and Shawn Colvin in particular, played relatively mellow sets. Now, I hate to sound like a hypocrite after the first rule, especially since I have acted like an absolute ape at many of the shows that I've been too, but she would be playing a really nice, introspective song, and this goof kept shouting out "YAH!!!!" at the quietest moments. Might be appropriate at Ted Nugent, but not here.
4) Let the artist know that you've read an article or two about her.
Probably the same goof yelled out "How's your daughter?" and other daughter-related bits. I'm sure Ms. Colvin was impressed. Or maybe, as my wife suggested, concerned that you might be a stalker.
5) Yell out song requests, particularly if it's someone touring behind their first disc.
There will be some exceptions to the rule, but in general, the artist probably will stick to that set list taped to the stage. Even those that mix it up generally play what they feel like playing, and not what you are bellowing about, even if you are cool enough to shout out the name of something other than a hit single. Yeah, I know, you once saw someone who seemed to actually listen and played something that was shouted out. That's not going to happen very often, so do shut it.
And if it's someone with one album out, and they're playing a 40-60 minute set (like when I saw Pete Yorn play at a bookstore here a while back), chances are that they're going to get around to most of those tunes.