I'm kind of conflicted about my choice to apply to so many programs. On the one hand, I feel that I should have applied to fewer programs (30 was way too much and paying for it was like lighting my money on fire and watching it burn). On the other hand, I got in and I don't think I would have been able to if I had not cast my net so broadly.
Definitely pre-write secondaries! Writing secondaries was the second hardest part of the process (the waiting was THE hardest part). I had a lot of late nights where I'd come home from work and do nothing but write secondaries for about 1 month straight. Ended up finishing writing most secondaries by mid August and gave up on a few because I was sick and tired of writing. Also, keep in mind that receiving secondaries may feel exciting at first since it's the first time where schools are now contacting you. Remember though, most schools send back secondaries because secondaries = $$$. Don't get overly attached to schools that send you secondaries.
Prepare to hurry up and wait. It's the nature of the beast. May/June/July, you're super busy preparing and submitting your primary then your secondaries. Then the waiting game begins. AVOID NEUROTICALLY CHECKING YOUR EMAIL EVERY 5 SECONDS! I did this. Every day, for months. I became so glued at the hip to my phone that I'm still having trouble weaning myself back off of technology.
Attend as many interviews as you can financially afford until you receive an acceptance somewhere. Then, and only then, can you afford to be picky about the interviews you attend. There were times I could barely afford to eat at while traveling to interviews let alone find a hotel. I even drove through the night to attend an interview in the morning, changed in the parking lot, attended the interview, then turned around and drove home because I had work and couldn't afford lodging. It was worth it though.
Have as much fun with this disastrous roller coaster of a process as you can. If the world is burning around me, I'd rather be that one guy laughing maniacally as he accepts it. He seems like he's at least having some fun.
Shrug off your rejections no matter where they come from. Just rejected from that big name school that you fell in love with? Go get an ice cream and put them in the past.
Celebrate your first acceptance no matter where it comes from. Just been accepted to that no name school you never heard of until you saw them in the MSAR? Go big. Scream, cry, do a little dance at work, then party. Realize that you have just achieved a major life event in getting into medical school in the first place should be rewarded. Treat yourself to something nice whether small or large.
Constantly remind yourself every day why you are doing this. The answer should be that you are doing this because you wanted it. Remember that.