To gain maximise bonus training for players, u collect all those greenpacks and dont use any until the next season. When the next season started then use all those hundred of greenpack and buy a bonus training (which need just 20 tokens) and powertrain them like crazy for a day...
U will gain 50% more upgraded players by using this method but it need preserverence and patient to colect and not use it in the meanwhile...
I use this method once a while and it provided me to maintain full squad with 7-9* players for forseable seasons... i think most prolific managers also using this method to maintain superior squad (which is full of legendary players)