Stop signing many players every season, get rid of any disruptive influences and when putting players on the transfer list try not to put 'not needed' unless you are really desperate for him to leave (whether that's down to him turning clubs down or because he's just a negative influence and causing trouble).
Instill discipline into the team, by issuing warnings and fines where appropriate such as for being sent off for silly or violent things or for missing training.
Start to manipulate players' morale and motivation more - if you go to the confidence menu, choose the 'players' option and see what the fans are saying. Warn players (appropriately of course - a minor warning or minor praise is the best way to start - save the big things for when big changes are needed) or praise them based on this feedback. Do not overuse it though - once every so often is enough.
Essentially, get a stable squad with a minimum of new signings every season and start to manage those players using the tools available.