Sneak up on an guard and use the Mind Drain power. You can almost fill your Psi bar up when it is completely empty.
Anytime you get to a checkpoint and are short on health or energy, save the game. Reload it to get full health and energy.
You can simply Mind Drain anyone to regain Psi Energy. You need to have at least a little Psi energy for this to work, or enough to get someone on their back. Use Telekinesis to throw him down and Mind Drain when he is about to get up. If you want a more gruesome death, wait until he is just about to get up and his head will pop, just like an ordinary Mind Drain at the end.
If you cannot sneak up to an enemy and drain him, throw him at a wall, object, or the ground. When he starts to get up, use Mind Drain. This will almost fill you Psi Energy completely if you are empty.
