I am a DO applying to both DO and MD EM this year. I would agree that step 1 is by far the most important thing to get an interview, period. I have been pretty surprised at just how competitive EM is, as I always felt it was a "middle of the road" field. Many MD places seem to be getting 1500-2000 applications for 15 spots and DO programs get 500-800 for 6 spots. You have to have something that sets you apart and it seems that research is essentially worthless in EM. I have a unique background via a previous career outside of medicine that has been talked about at length in all my interviews and I think may be the one thing that PDs will remember about me. I would advise anyone to have something of interest about them that sets them apart from the rest of herd that is completely outside the world of medicine. Showing that you are a complete person with unique life experiences makes them like you. Who would you rather work with, a guy who sat in the lab and looked at a microscope all through college, or one that traveled the globe/started a business/played the drums in a rock band?
The other thing I wish I knew was just how little time you have to pick a field. I bounced back and forth between ENT, surg, and EM all through school for no real reason other than I liked the "idea" of being one of those types of doctors. I expected that step 1 would cut my options down but when I did pretty good on it, I had to make some decisions. For me, I decided that I had to give one field all my energy. I don't think I could have applied to two of them and been a quality candidate. And because there are only so many golden elective months, you have to decide early. Best advice I got was to pick two by December of your 3rdyear, and then set up away's in both. By 4th year pick one and run with it. Audition months fill up crazy fast in small competitive fields like ENT and Optho and even EM.
Being a DO has been a mixed bag. I feel that all of the MD programs have been extremely open and honest about their opinions of DO's, some mind, some dont, I certainly can't fault them for it. I only applied to places that had taken a DO recently and I think that is the best course. Also, I have been very surprised with the quality of most of the DO programs (all but one of the 10 I went to have been solid), however, geographically speaking the DO programs are pretty crappy (MI, OH, PA). Deciding to drop out of the DO match has been hard and Im still not sure what to do.
My mantra would be, study hard, pick a field early that you enjoy, and don't be a prick.