I just got back a few days ago from a JMT thru-hike. I hiked the entire trail (more than 220 miles including the trail out to Whitney Portal) WITHOUT A SINGLE BLISTER.
Some of you may be thinking, so what? But for me this was a big change, as those who have hiked with me in the past can attest.
So, what's the magic formula? As the book Fixing Your Feet makes clear, what works for one person doesn't necessarily work for another. That said, here are the changes I made from previous hikes:
1) Minimalist footwear lifestyle: For the past 2 years I have worn nothing but minimalist footwear when I am not backpacking on long trips. I wear Vivo Barefoot Dharmas to work (one pair black, one pair brown gets me by), and all my casual shoes are minimalist. I also day hike and do short backpacking trips in minimalist shoes (I've tried many brands.) I suspect, but cannot verify, that this has been the biggest contributor to my no blister trip. My feet have toughened enormously and have calluses in all the right places. My ankles and feet have sprouted muscles where I did not know feet had muscles. This has prevented the kind of soreness and other nagging injuries that sometimes cause you to walk just different enough to create blisters over the long haul.
2) Shoes that fit: This should be obvious and easy to deal with, but as many of us know it's not. We think our shoes fit great until we go on a big trip and little issues become big issues. Here is how I ensured good fit. Last summer I went on 2 50 mile trips in New Balance 790s, the predecessor to MT 100/101s. No issues with blisters, so I bought a few pair and stockpiled them (knowing they would be discontinued), saving one pair brand new for the JMT.
3) Sock variety: On this trip I brought 3 pairs of socks, each one a different brand. At each rest stop / water resupply I took off my shoes and socks, let my feet dry out, and then put on a different pair of socks. Thus, my feet were never rubbing in the same way for more than a couple hours at a time. Not sure it matters which brands, but I used Feetures, Wigwam, and Wright socks. All lightweight running socks, though the Wigwams were much heavier than the other two, so I tended to use them on uphill climbs to avoid my foot jamming the end of the thicker sock into my shoe. At mountain passes, I switched to the wright socks or Feetures for the downhills.
4) Gaiters: I used Dirty Girl gaiters last summer when testing the 790s and used them again on this trip. Invaluable to keep heavy dirt and grit out of my shoes, but not bullet proof. On the JMT, there is just too much scree to expect to keep it all out, especially when wearing mesh trail runners.
I do not know what proportion of the blister free experience came from each of these methods, but I just thought I'd throw this out there for those who have had blister issues in the past. Having no blisters certainly made the JMT a much more enjoyable experience than it would have been with constant foot care issues.