You can also improve a player extra by assigning a specialist to train him individually on top of the tri-weekly training sessions.
To give a player extra personal training go to his player profile and click the "Extra Traning" button just below the player's attributes.
Each extra training gives a player 10 TI (or 10x 0.5 skill point improvements).
Note that the 10 extra TI a player gets from an extra personal training is ADDED on top of the players born potential. It does NOT mean that a player just faster reaches his born potential. No potential is lost by giving extra training.
A consideration though is that by given a player extra training he will faster reach higher skills and hereby his natural developement (born potential) can be hit by the hill-shaped training.
So the trade-off is: Wait to train extra until he is fully developed and not be hit by hill-shaped training faster, or give the player extra training when he is younger and make him be stronger at a younger age and have more years in FMU as a better player. It will also be a good idea to give extra training to a player before selling him.
When a player is sold/bought his extra training is kept, but the buying club is able to train the player extra again starting from the minimum Pro rate per extra training.