This website has been so helpful for me during the application cycle as I was kind of assessing where I should apply. I thought I'd pay it forward by sharing about my experience.
Overall, I feel really, really lucky. I had pretty solid numbers & a decent amount of hospital volunteering experience, enough to tell a few good stories about patient interactions in my secondary essays. I think those two aspects were what won me a surprisingly lot of interviews - otherwise I don't feel that I am very outstanding. I expected far fewer interview invites.
The most difficult part was definitely SECONDARIES. I got started on my primary application and main essay pretty early on, so that wasn't too difficult. But cranking out essay after essay in order to return secondaries ASAP was difficult. My advice is:
(a) PRACTICE WRITING EARLY. If you're a science major like me who throughout undergrad would prefer taking another organic chemistry test than write a paper, do this. You get better at writing and words will come out more easily the more you practice. Before the application cycle even begins, start practicing. Do Reddit writing prompts. Read articles and write summaries for your friends. Write a short essay about something, anything, every few weeks. This really helps you be in tip-top shape when it actually matters and you have to churn out a ton of essays quickly.
(b) Don't aim for perfection. Of course, you should most definitely proofread and make sure there aren't glaring errors in anything you submit to admissions. But don't get caught up on an essay and try to perfect it. Once it's decent - it tells a story, makes sense, and a couple people read it and can follow your train of thought - let it go. There's no point in pulling out your hair over making it the most compelling thing ever. I got interview invites from places I gave really, really, mediocre essays, and I got no-invite-rejection from the school I gave my most favorite-bestest-most-awesome secondary essays (Vanderbilt. Grr.)
(c) Don't stress out about turning around all your secondaries in less than 5 days. When I was applying, word on the street was that YOU MUST DO THIS or else your chances drop drastically, but I didn't find this to be true. I ran the stats on whether secondary turnover time was significant in determining whether I got an interview. It wasn't. Don't sit on your essays forever, but don't stress out about turning them over in a couple days, either.
There's a lot of resources on SDN, especially interview feedback. Use it. I usually reviewed the questions on SDN prior to my interviews because it gives you a sense of what questions might be asked. Of course, you might get thrown a curveball and interview with someone really tough/weird, but I generally found it to be helpful, at the very least for calming my nerves by knowing what to expect.
I really, really recommend staying with a student host if you can. The ones I encountered were all SUPER nice and awesome people, and really willing to answer your questions and give you tips on the interview. I think the possibility of meeting someone who can really help you out (increase your chances of getting in by giving you a lot more info about the school that you can use in your interviews) is definitely worth it.
It's a tough, long process, and the waiting game is rough, but you can do it! You will be great future docs. :)