Best way must be to focus on one or two positions and make sure you got players to train there. You should have young (preferably 18 year old) players that preferably got some potential. That will improve your team in the best way. For other positions you could either buy older good players which are rather cheap and still at a high level or keep your current players if you want to keep a high team morale. You can sell one or two of your trained players after a season or so and strengthen other positions.