Just the standard advice: write your personal statement now, get your transcript requests together (especially if you have multiple schools) even if you can't send them out yet, etc. Aside from the much reported "it helps your application to get it in early," it'll also help your anxiety levels. Imagine how nice it would be to know in OCTOBER that you got into medical school!!! I didn't know until January, and the intervening months when others were getting accepted and I didn't know were acutely painful. Oh, if you don't already have your letters of recommendation together, now would be a good time for that -- ask for about twice as many as you need, then pick and choose which ones to send if by some miracle you happen to get all of them!
Don't try too hard to make yourself stand out -- stay away from gimicks. Keep in mind that medicine is a pretty conservative field, and one of the questions these people are asking is, "Is this a person I would trust my dying grandmother with." Also stay away from sensitive topics to the extent you can (like politics and prior histories of alcohol abuse, etc.). Don't include anything in your application that you aren't comfortable talking about at length.
Whenever you will be interviwing (fall or spring term), run the fact that you're going to be interviewing by your professors early so they understand when you say, "I gotta go for an interview, can I make up the midterm." Some profs have a really hard time with it and get bent out of shape, but most of them will tell you that if you approach them early in term... so give them a chance to get you that information.
Be nice to everyone at your interview. Thank the people who organized it on the way out -- they won't be on the receiving end of your formal thank you letter, but will greatly appreciate the fact that you noticed their level of commitment to making the interview day smooth.
Keep in mind your first interview won't go as smoothly as later interviews. Just take it in stride. Be willing to stop and think about questions you don't already have answers to. Don't sound to well rehersed (even though you need to have your act together, people have gotten dinged for sounding like they had their "why I want to be a doctor" speech memorized. At later interviews, try to remain enthusiastic even though you are thoroughly sick of the entire process.
Keep in mind when you fill out your AMCAS that you'll be footing the bill for any travel to and from interviews, and think about how much your pocketbook can handle. There are people who can spend >$5k on travelling to and from interviews. I, for one, applied only on the east coast and the single school I interviewed at in the midwest cleaned out my meager savings (U Chicago, which was worth it, but *ouch*). You might save a whole lot of money by applying based on geography.
Start looking now for a suit if you don't already own one, to give you time for finding a good deal. Get it altered professionaly; it will look almost taylored. Get the person behind the counter to help you pick the current fit if you aren't sure. Nothing is worse than an ill-fitting suit. Black with cream or white shirt and a red, conservatively pattered tie will go anywhere for a guy. For women, black skirt suit or (not my personal favorite, but seems to be allowed...) pant suit.
What else? Keeping organized will help you stay lower stressed. Again, apply early -- if not because it improves your chances, because you will find out earlier where you stand.
I wish I would have applied early decision to my top choice school... it would have saved a lot of money (it's where I'm going next year anyway).
Best,
Anka