This past cycle I submitted 37 secondaries, so I have a few ideas (10 to be precise) about how to stay on top of this.
1. Perfect the primary. Personal statement and activities sections went through many drafts, were seen by many eyes. I was done by mid-May.
2. Get school list in order. Consult advisors, family. Find out what is really important to you. Make sure you have at least $5K to spend.
3. Identify schools on your list with consistent secondaries. Get several years of data. Some schools change secondaries every year (e.g. VCU, UCLA) while others haven't changed for nearly decade (e.g. Harvard). Prioritize the ones set in stone. Save the mercurial ones for last. Personal preference for post-screen secondaries like Vanderbilt and UCs. Guess who wrote a 10,000 word autobio for Vandy but didn't get a secondary? This guy.
4. You're going to want to start writing secondaries for your "reaches" immediately. Don't. Mix it up. Low tier, high-tier, mid-tier, low-tier. Your writing will get better and more efficient with time. You'll hit your stride by about a dozen secondaries. That "sweet spot" is when you should probably write secondaries for the schools you like most. Practice makes perfect.
5. Do your research before completing each secondary. I suggest buying MSAR to start. Do a "deep dive" on official school websites too. Use the "Notes" section so you keep a record for "Why this school?" Will be useful for your interviews too as a quick reference.
6. Don't be afraid to work on secondaries simultaneously. Sometimes you get inspired by one secondary but just can't get it together for another (e.g. the Duke/UMiami/Loyola time-suck monster).
7. Good writing is good editing. Pump out those secondaries to a few good women (and/or men). My girlfriend nearly read every secondary (yeah, she's great). Having a "mutual" secondary agreement with another applicant can also be useful. Just know you have to factor in time for proofreading someone else's work.
8. Secondaries start rolling in but you aren't finished yet....it's GO TIME. Aim for a 2 week turnaround for most schools, especially for the ones with rolling admissions and an early interview season (e.g. Tulane, UMichigan, NYU, Mayo). Pay attention when schools give a deadline. Otherwise, feel free to work longer on your secondary (3-4 weeks) for the likes of Harvard, Yale, Columbia, etc. A later secondary hurts you less at non-rolling schools. However, be forewarned. Admissions may not be rolling, but interview slots always are!
9. Inevitably, some schools will change their secondary for the first time in years. (Looking at you Yale and Drexel). But you already wrote it! Breathe. That's the risk we take. Fortunately, you've already got 20 secondaries worth of material to adapt and reuse!
10. You will burnout. For me, it was at secondary number 25. For you, could be more, could be less. But rest assured, it will happen. This is part of why we start early. When you feel the burn, then you can back off and return later to do your best work.
Bonus: Be organized. Excel, Word, Number, Pages. I don't care. Just use something. Make it color-coded. List "hard" deadlines, "soft" deadlines (e.g. 1 -2 weeks) List submitted or not submitted. List current status.
Using these guidelines, I managed to write a secondary every 0.5-2 days early on, but that stretched out to as many as 10 days (especially during GO TIME). In the end, every secondary was submitted anywhere from 10 min to 10 days after receipt with a peak time at 2-3 days. And yes, there are tertiary/pre-interview essays (e.g. UMichigan, UCincinnati). Don't bother with those until you get an interview.
Godspeed.
