the best for me is training in 3 bars, only the good/fast trainers but individual - not 3-4 or more players together.
Use some greens to earn some good sp (20,30 or more) in one day and then let your player rest for the next day.
Friendlies are good for my team lv18, where I have a full stadium of 260 K' and earn 10-12 M' from one game.
it didn't help me so far to have any conclusions about formation, tactics or the specialists but Khris saying that he can find them (the free-kick and the corner specialist I mean).