Keeping the squad happy

I've had a fair few seasons with Peterborough now and have managed to get them to the premier league, I've managed to get a good team together.

I'm having problems improving my squad. When I got promoted to the premiership I bought a lot of experienced players that would walk into my team, but I also need a way to keep my now large squad happy. Anyone got any tips as it happened when I got to the Championship too and in the end I had to realease a good chunk of talent because of poor squad harmony, I'm now competing in the top flight but have again got this problem about lack of squad harmony. Is there anyway to keep these people happy, if not, how do I convince good players to join my club as a squad player as they're never willing to come.