In my experience always carry your pen light, tongue blades and gauze packs in your pocket, it's always helpful to have them on hand when doing rounds with the residents, if you think they might need something grab it, and have it on hand; try to see what pattern they follow and anticipate their needs (IE, do they do hands off exmaniations of the patients in the hospital, or do they glove up and check them all, have the Pen light shining before they ask for it, and the tongue blade popped open and extended as soon as it looks like they're gonna do an examination). Comfortable shoes are a must, long hours standing and only watching is terrible for the soles.
Read up on all the faculty and residents before hand, show up with some idea about what the program does and what type of cases they do (Cancer? Trauma?) Be familiar with the instrument names, at least the extraction kit, in case you're assisting and they ask you to pass the Seldin, or something you'd better know what it is. Don't be over bearing to the residents at busy times, try to be tactful when asking questions about your own position and what to aim for (GPD, scores, etc), try to get to know the residents first, ask where they graduated from, find some common ground etc.
You'll probably be expected to take call a couple times with the residents (usually it'll be a 1st year on their own unless something big comes in), this is a good chance to get to know them, they're still fresh out of school and will likely have the most compassion towards you. This is also another opportunity to learn a lot, especially if you're seeing any ED treatment of trauma (facial lacs, mandibles, infections etc); and you'll likely be able to help a lot more in these situations since the interns usually have no one with them, be prepared to help set-up some quick Lac kits (sutures, iodine swabs, gauze, gloves, suture kits etc); since if you're at a busy hospital you can go through 3 or four of these a night easily.
It sucks that you're expected to know a lot more than you have any right to, and you're supposed to be everywhere while also being out of the way, but that's the nature of an externship; personally I found them tedious towards the tail end, (especially the hands off ones), just because there's not much to do besides stand and watch. But they're a good way to see the life of a resident, and get to know a lot about programs you're interested in.
It's not a requirement but it doesn't hurt to get something for the residents/faculty as a thank you on your last day (cookies, donuts, etc), just as a thank you.
