Brush up on your crash airway skills.
Just kidding. There's some great advise from the masters above. I'll throw in my 2 cents.
What Wilco said is right on. The trick is knowing which questions to ask which patients. It's quite algorithmic. There are 3-4 questions that are essential for each chief complaint beyond the usual H and P stuff. For instance, in a r/o ectopic you should ask about vaginal bleeding, fertility med use, Ob/gyn history. It sounds simple, but believe me you can forget. And you look pretty dumb when the attending asks "so did the patient have any vaginal bleeding?," and you hadn't asked. Same for minor trauma and asking about the tetanus shot. Hand injury - there's a few physical exam things they'll always ask you about. Headache - oops I forgot to do a neuro exam. Back pain - How's their gait? Oh, I didn't actually walk them. So I think running through the agorithms for each common complaint would help - or would have helped me anyway.
Another thing is being able to exude confidence while you give a 3 minute presentation without eh, um, uh, pausing alot and diving right into your differential then you plan. It should all flow fast and nicely without forgetting major portions. It should feel more like "hey I just saw this patient in room X, this is what I thought was important/remarkable, I think it might be A or B, this is what I'm doing, how does that sound to you?" This is easier said than done. It's hard enough to just get a decent H and P from some of our patients but you've got to go beyond, make it sound smooth, and have the loose ends tied up. Do the chart biopsy before you present. Have the old EKG in your hand. Know your renal patients last Creatinine. Otherwise you'll be sitting there while the attending looks it up on his own. All this little stuff adds up.
If you want to do procedures like central lines, intubations, reductions etc - you should read up on them in Roberts before the opportunity arise. When you ask "hey can I do that central line," people love to say "sure, have you read about how to do it?"
The more reading you can do in an EM review book (like First Aid or whatever you like), the better off you'll be because once the Sub-I you are being evaluated from day 1. Fist Aid is decent. If you don't like reading, ACEP-EMedHome.com has some good lectures for cheap.
Don't see too many patients. This was really hard for me. In my experience, interns and junior residents were impressed when I saw a lot of patients; senior residents and attendings were not. Every patient you see is more work for your preceptor. They generally don't mind, but if you miss something or aren't around to fill out the discharge paperwork because you're trying to juggling to much then you're making even more work for them.
Oh yeah, and hop in on the non medical conversation. Act like a real person not just a go-getter.
The nurses should know you and like you. EP's trust and respect their nurses.
Always have trauma shears.
Well, I hope that helps. Maybe you can learn from some of my mistakes!