Ok, children. Here is advice from a resident who has switched specialties a few times and knows a thing or two about matching and getting into different specialties. If anyone has questions, please ask.
First, do well in school. Work hard and do your best. You do not have to be perfect. Don't stress, just study, and do your best.
DON"T BURN ANY BRIDGES. This is a small field (all of medicine MD or DO). One a hole can ruin your freakin life. I mean it.
Study for your boards and do well on them. This is by no means a sign of intelligence, but they don't care. They want people that they can "predict" will do well on their specialty boards, and the only way they know this is to look at your board scores.
Absolutely positively DO NOT FAIL ANY BOARDS. Many residency programs will disqualify you from their interviews if you fail any step, especially Part I. Failing Part II and III don't look good because they are more clinical. Take these seriously, but don't stress come test time. Relax, and do your best. STUDY. I don't care if you are on surgery rotation when you have boards. They won't give a shiite either. That is the sad fact. These boards DO matter to competitive programs. Some of the smartest people I know don't do well on these tests, but that is just too bad.
OK Boards are important, but do you know what has always got me the residency I want? You need good letters of recommendation. The quality is more important than who you get them from, but if you can get a great letter from a bigshot, you are golden.
Make real friends in this field. Surprisingly they will be there to help you out in your time of need. Don't use people. They will see through it eventually.
Don't ever ever talk bad about doctors you work with no matter how tempting, and no matter how big of an ahole they are. Things get out, and you can be screwed.
Make sure you really know what you like to do. Take it from me. I have been in a few different specialties. Put the time in to really find out what they will be like WHEN YOU GET OUT. Don't worry so much about what they are like in medical school or residency. Residency sucks in many ways, but you must keep your eye on the big picture. Money is important. I like it, but you have to do that job everyday to. If you don't like it, you will dread work forever. Money will ease the pain, but you may regret your choice. Do what you like to do, and you will most likely be successful.
Never quit a residency. At least finish out your contract year, or find another position until you do. You will be screwed if you just quit. Don't disparage another profession just because it wasn't right for you.
Don't talk back to your attendings. Worse thing you can do, especially in front of people. I know personally. I was badgered one day when I was having medical problems for not holding the gosh darn lap camera still, and I finally had enough. He was out to get me after that day, and ripped me every chance he could throughout the hospital system. Small man, yes, but they are out there.
Try to have a life and take care of your relationships. Residency almost ruined what was most important to me.
Take care of your health. Take it from me. Excercise and eat right. Hospital food is bad for you. Watch what you eat there. There are more healthy choices now.
When you do apply for residencies some day, apply early. Many places will invite you before they get your Deans Letters. Get your letters of recommendations as early as you can. Doctors are busy, and will put these off for weeks. Trust me, I know. You can't piss them off though.
DOs: If you can take the USMLE. There are still some programs out there that don't take your test. There are even some programs that used to take your test, and now only take the USMLE like anesthesiology because everyone wants to do it now.
I know these seem rigid, and there are exceptions of course, but you will have more problems. This is advice from my personal experience and my friends. Any questions, I'll be checking.