Just like dermography says, be enthusiastic about everything. Look for any opportunity to be helpful to those around you. Pay attention to all the extra crap that your resident is having to do: getting the paper work out to get the patient's consent, going to get the cryo gun, grabbing a camera/photographer, looking for their pen. If possible, get these things ready for them in advance. Find out what size gloves they wear and get them ready for your resident. Be quiet. Ask questions when there is downtime and shine WHEN you are given the opportunity. Don't be a gunner or an *******. If you do all of these things, people will miss you when you are gone because you were a team player, eager to help and eager to learn. Even if you don't match at an institution where you did an away, derm is a tiny world and you will see these people around if you are lucky enough to match into the specialty.
Away rotations are a fine balance and the ones that do the best are ones that are genuinely nice and helpful. Being smart comes second on an away rotation. A lot of an away rotation is how you do things. A lot of life is the how and not the what. So what do I mean?
1) Take a chill pill. If you take everything too seriously, it shows. That rarely ends up well.
2) Be aware. There's no point in restating dermpom's point because it's dead on...read that previous post carefully, especially the part about paying attention to what might bog the resident down. In the past, I've had med students do something so simple as have a pen ready for me and by the end of the clinic, I had the warm and fuzzies because they had been so thoughtful to keep the clinic moving along smoothly. It meant so much. If you help with the little things like getting the consent ready, getting the cryo, helping facilitate the biopsies with either positioning the patient or getting the pillow or letting the nurses know, etc. goes SUCH A LONG WAY. However, if you are intense or come across like a suck up then it turns people off so you'll have to find that balance.
3) When you ask questions, make it a good one. Again, this is the how you do it. Remember how people say there is no such thing as a dumb question...well that's wrong. Sometimes you can ask dumb questions and usually its when you seem like you are asking a question to just ask a question. Make sure you care when you ask.
4) Look things up if you see an interesting patient. This is especially true if your attending and resident have a prolonged discussion about the patient's clinical scenario. We just don't have time to look things up on the fly and so if you are able to look it up during a period of downtime, that goes a long way.
5) Be assertive. If you are a wallflower, no one will remember you. This goes back to being a part of the team and being actively engaged. Ask about any opportunities to work on publications. Many times attendings or residents will have hanging projects that need help to finish up. You won't know if you don't ask.
6) Smile. It matters. Everyone plays of body language first before they talk to you. If you come across as a sweet person, it's a huge deal. Even if you are having a bad or stressful day, remember to smile.