Just a few thoughts for the OP:
1. Work hard but don't overestimate the importance of class rank. By all means...do as well as you can, but pre-clinical grades are somewhere around the 5th or so most important thing in an application.
2. Board scores are pretty big. With that in mind, begin using board review materials early on for course studying. Obviously, most of the board review books don't go into enough detail in biochemistry (for example), but they do hit most of the things that you'll see on your course exams. No matter what school you attend, the same concepts and facts are key no matter what and will be tested on exams AND boards (e.g. Starling curve or the coagulation cascade).
3. Get involved in things YOU want to be involved in...not just because you think it might look good later on. Feel free to do research in ANYTHING you want to. Do intramural sports if that's your bag. Hanging out with an OB/GYN once in a while might not be a bad idea, but stay diversified!
4. Getting good (and meaningfull) letters will probably come during your fourth year. During that year you'll want to schedule elective rotations in OB/GYN at places you think you might want to do your residency. You'll want to make sure that these electives are done at respected institutions with respected academic and/or clinical faculty. Those folks will write your letters and that may be the most important part of your application (besides your interview day, perhaps). Obviously, if you attend a school that is affiliated with a great OB/GYN program, then you can get a letter during your required third year rotation.
5. Nobody gets into anything that's fairly competitive without doing well during your core clinical rotations. Be sure to do your best on every rotation during your third year. Remember that most rotations only last one month so even if you hate the service you are on, it's only temporary and ANY medical student can fake being happy and helpful for one month!!
If you have any further questions or comments, feel free to PM me.
VALSALVA