All good tips, but nobody has mentioned one (1) thing: keep the transmission in neutral and use only the handbrake.
I didn't know any better, and once when towing my Healey between Breaux Bridge and Shreveport, Louisiana back in 1986, I left the XMSN in A-gear, and the constant rocking broke one (1) tooth off of the pinion gear. Fortunately (...?...) it happened on the way home, after an autocross event, and not before. First time I drove once it was off the trailer, it was immediately obvious that something was wrong, just making a different noise from the rear end. I know it didn't happen at the event, because we were driving the car around S'port afterwards, and besides, if it had broke that tooth off while under (a driving) load, it wouldn't have just fallen to the bottom of the case__there would've been collateral damage!
Flat-tow your cars with the XMSN in neutral, and (a well adjusted) hand brake pulled up.