In my experience there is no substitute for real games, which is why I will always send them out on loan after they reach 16 or 17. So long as the club has good facilities they will make progress. But before then I make sure they are tutored and that they are set on the right path. If you are going to keep them at your club, the main thing is to keep them match fit and have a good player with a professional personality look after him. But you have to spot the stars early and be pro-active in their development and not just sit them in the under 20s for a few seasons and see what happens. Not only do you have to improve their stats, but also mold their personalities.
However, not all players will reach their potential. They could have a 5 star potential, but if they don't have the determination, ambition, fitness or work ethic, they will not improve much no matter how you try and motivate them. I had a German striker a in FM13 that was totally obnoxious, lazy and had no ambition. No matter who tutored him he fell out with, no matter how often I spoke to him nothing happened. This guy could of been a legend, but ended up playing in the German second tier.
But in 90% of the cases there are ways to get even the most awful personalities to improve via tutoring and having a word with them every now and again. People under-estimate the power you have as a manager to mold a young players personality and form by just having a word with him every now and again - just make sure you have cause to talk to him first.
Micro-managing training will only improve their overall stats by a small margin. The biggest factor in developing by far is moral and games. Keep them happy, keep them fit.
