Every OJTI knows (or should know) what standard is required to validate at their unit or sector, additionally they also know that the learning process doesn't stop at the validation.
Encourage the trainee to make their own decisions from day one, but make it absolutely clear who is in charge and lay down the parameters beforehand.
Lots of encouragement and constructive critisism go a long way in building confidence, equally, the ability to put an over confident trainee in their place quitely and firmly is a pre - requisite of the job.
Shouting and sarcasm doesn't work.
As for chopping a trainee - sometimes it is necessary for the good of the student and the unit, it doesn't always mean they'll never work in atc again, just that they aren't suitable for that unit or sector.
Extra pay
If only...............