OK, some words about getting mind-controlled:
1. As said above, PSI-Strength is an UNCHANGEABLE value. You can`t higher or lower it by no ways.
2. This is the value which defines if a soldier is mind controlled or not.
3. Aliens only mind-controll who they see! So there is no problem in taking PSI-weaklings in etheral bases or to Cyd. Just hide them carefully and let them sniper out of the Avenger. (That is actually the point why taking them with you, if they are no sharp-shooters with an aimed-shot percentage above 100
just get rid of them.)
4. 2-4 good (and I mean very good!) psi-troopers who can mind control the aliens are far enough to make the largest UFO of any race completely clear, bases and Cyd as well. Just search for aliens with either a soldier with a high (80 +) Psi-Strength or a (hoover)-tank and controll the aliens. This brings their morale down very fast and you might witness a crazy Etheral who blows up a blaster bomb in his own battleship.
5. The most helpful rule again: Don`t show yourself, don`t get controlled - show yourself, get controlled.