Here's an excellent strategy how to play this game, I'm using it.
At the beginning of every season you should replace three your oldest players: buy three nice expensive fast-trainers in the Market, 18 y/o, 4-5*. It is good if they have two postitions or SA. It might not be an easy task, I woke up very early morning (when server resets) and sometimes got myself into a bidding war. You don't have to buy them all in one day, but the sooner, the better.
Power-train them new positions (if needed), SA (if needed) and only after that skills (stars), do it for the whole season. Spend your green pack only on these guys. It is probably a good idea not to let them play, because the old guys usually do their job better than this fresh meat, but it's up to you. It is possible to get them up to 7* if you do everything right, squeeze green packs out of everything and use them only on these FTs.
When the next season starts, buy another three 18 y/o FTs. In four seasons you will have a dream-team of young fast-trainers. Replace players when they turn 22, because at this age they don't get as much skills as young players.
These recommendations are not carved in stone. If you can't find three guys, two might be enough. If one of your players doing exceptionally well on the field (his ratings are high), you probably should let him stay for a season or two even if he's old.
Also, here're probably interesting reads. A lot of micro-management though.
A little trick how to "enhance" the level of a drill: https://forum.topeleven.com/tutorial...tml#post436134
How I choose drills for a player: https://forum.topeleven.com/tutorial...tml#post436876