We never really run out of candidates. The good ones, that have just the right amount of crazy, are however hard to find.
We have increased our qualification rate (at least here, in Montreal) somewhat by reassigning some trainees to another sector when it looks like they might not make it in the initial sector they were assigned to. Even though they might be headed for a wall in a certain unit, their kind of crazy might work best in another type of control. Terminal is one thing, high level enroute another and low-level-non-radar is, as well, totally different.
Having the trainees paid right from the start is good all around. Some thought that only those ready to do it for free would be motivated enough to do it but what we ended up with was often some kids living at home with mom and dad paying everything and packing them lunches or some having to have a job on the side to pay rent and washing out simply because they couldn't keep up. Either way, it wasn't working. We have worked and continue to work on our selection process since that is the key to finding the best candidates for the job.