Great thread. For me personally I try to do the following.
From. 15-17 I focus on tutoring them. give the younger players a weaker tutor ( allthough one who is still better mentally then the youth player). This will slowly improve and mold their personality. By 18 years old most youth players who have a future with your club should be at least have balanced personalties. This is essential. At 18- 21 I like to send a youngster on loan to a a lower club, however they must meet strict criteria as mentioned previously in this thread. Ensure that they will get games, that the club has a good standard of training facilities and that the club has adequate coaches. Many Lower clubs do not have fitness coaches or GK coaches, bare this in mind.
When you agree to a loan keep track of the player. If his average is around 6.6-7.1. This is optimum. The level is not to hard or easy and his morale will stay high, morale is important to improving attibutes. If the are not playing games, recall them and try and find them a different club. If all goes well, after 6 months you have a decision to make, either recall or let him continue. I always 1st check to see if attributes have improved, if they have then a extra few months on loan could help them. If no improvement can be seen recall them, the training facilities may not be good enough.
If you do recall, this is a prime time to start giving the player a specific training regime to mold them and give them an specific attribute to improve. You should now get your best tutors on him. try and get that determination level up and improve their mentality from balanced to determined, ambitious or proffessional. Also give him 1 or two cameo FT appearances. You should find your players improving at a very resonable rate. You can turn this into a nice production line. I like to have 2 youth players in each position, so that while one is out on loan the other is at the club being tutored. This is great for lower clubs who may only have 1 viable tutor poor finances and a weaker coaching setup.
Most players who are going to be great players will naturally become good players, regardless of what you do, however this process can turn a good player into a more specialised/ moulded player who will be able to overachieve because of personality and a specialist like set of attributes.