Pretty much this:

power training:
Buy or save up a ton of Green Boosters and have maybe a third as many Red Boosters.
Set training to hard intensity on the (fast training youth!!!) player and keep training him until you get to the skill/quality you want or when you run out of boosters.

maintenance:
What I have been doing for training to try to keep guys' Q up with level is when they are 50% toward their next point I give them extra training, also I play the older guys or slower trainers in every match possible.