Before moving another's scope, be sure to ask whether there are clamps to release, how to change eyepieces, and which way to turn past the meridian. Some have electrical lines running up to the tube and so cannot be rotated arbitrarily. As you stroll about the site you may find someone with an eye glued to the eyepiece, never moving, holding a box with some buttons and a stretch cord. Never walk in front of the instrument. If your tentative hello is met with an enthusiastic greeting, you've found one of the former variety.
Few people realize just how chilly it gets late at night, even in summer.
So bring about twice as much warm clothing as you think you'll need. If summer, a light and heavy jacket, warm shirt, jeans. Wear sturdy shoes - most sites are irregular and the pebbles get to bare feet. The greatest heat loss is from your head, so you'll want the hood on your coat, too, especially if there's a breeze. You'll need extra energy to remain effective through an observing session, so you'll want to bring along plenty of sandwiches and snacks, and coffee or soft drinks.