Other things I'd add (and wish more people would take note of):
Turn off the internal lights in your car, including the boot light. If the boot light doesn't turn off cover it with some red cloth or the like.
Have a
small red torch. Don't turn up with a Dolphin lantern covered by just one bit of cellophane, that still waaaay too bright. Dont use white light!!!
(As mentioned above) Dont' turn up with the high beams on and blast the whole observing field. If you're not familiar with the site arriving in daylight is recommended.
If you're going to leave early place yourself near the exit or facing the exit so you don't blast everyone again on your way out.
If using a laptop make some effort to at least shield it or using a filter over the screen to dull it down. I'm not much of a fan of the red 'night vision mode) some things have, I still think its too bright (yes, I'm a bit old school when it comes to light on the field, the less the better).
Most places I've been its been tea and coffee galore but alcohol to be consumed quietly if you really must have it and do not ever get drunk. You're there to do astronomy, not drink. If you're a serious observer best not to have any as it can affect your night-vision. Best to check with your local group on their policies, however.
Don't play loud music blasting from your car speakers. If you really must have music use earphones. Not everyone will share your musical taste, unfortunately.
Bring a chair, unless you like to do lots of standing.
Some of this might seem like common sense, but there's always someone who doesn't see it that way.