Good subject this one! Question first. How many OJTI's of have you seen training people that shouldn't be OJTI's in the first place? I have seen too many.
A good OJTI, in my opinion is almost part psycologist. The first thing that needs assessing is what type of trainee they are to begin with. An OJTI can't just train trainee 'A' the same way as trainee 'B'.
As always, a good OJTI has to be good enough to allow a situation to flow/deteriorate to expose the learning points without compromise to safety or expedition of traffic. The envirnment also needs to be geared to accept what is going on. By that I mean some people don't like to give the latitude required in the training environment. Some centres don't have appropriate sim training which is a real pain. But those that do are so much further ahead. I have never failed in getting a trainee up to speed except one. I did not recommend this person but it was taken out of my hands by management and they had them rated about two weeks later after training with another OJTI - the type that shouldn't be one. 2 months later the newly rated person had a major incident and is no longer a controller. So there you go.