Many people have left a star party earlier than they wanted to because they just got too cold. If you might stay late, bring along a snack and something to drink. Try to arrive before dark. It takes 20 to 30 minutes for your eyes to get fully dark adapted and only a second or two of white light to make you start over again. Don't use white flashlights for the same reason.
Red light has almost no affect on our night vision.
There are often wires and power cables connecting equipment that are easy to trip over in the dark. Avoid loud and boisterous behaviour. Star-gazing is a quiet, peaceful activity. Finally, don't be afraid to ask questions. The only stupid question is one that is not asked.