Great question. You will get ample time to develop all areas of emergency medicine during residency. But if I had to pick some areas, here we go:
1) suturing/wound management
2) acls/atls - at least the algorithms
3) ekg's - it's going to be tough to be proficient at this by simply studying - no matter how much Amal Mattu you listen to, it only gets reinforced during daily repitition - however it's so important that a basic understanding is absolutely essential
4) ultrasound guided IV's - I don't know if you would have any opportunity to practice, but if you come in able to knock these out on day one, you'll not only be extremely useful on all rotations but you'll have some quick respect from the seniors
5) basic ultrasound skills - have a good understanding of your way around the machine, as well as all the basic scans