Here's another rich seam of OSR material my mining of the old school blogs has turned up. I suppose I should sort all of these links and ideas into a structured and coherent guide at some point.
Hack & Slash goes into the Quantum Ogre and why Illusionism is bad. The Alexandrian explains what Abused Gamer Syndrome is and how to fix it.
The Blorb principles @idiomdottning.org are for prepping old school adventures.
Then @SpiderQueenGaming, Ed, has some OSR wisdom here:
And some more practical advice here:
Finishing off with 8 Lessons from running a successful sandbox.
This is an interesting addition that came up on Reddit. A list of geographic features used in the lord of the rings and their meanings. Very handy for sandbox place naming. The OP says they are doing one for man made features as well.
I also found this interesting blog. This very detailed and analytical review of the The Black Wyrm of Brandonsford adventure is helpful for when designing your own adventures, or just improving published ones.