It is very hard to achieve but using the right training such as intensive motivation, the right balance of players in the right formation (so everyone is happy with playing/not playing) and good results can make it full but beware chopping and changing your squad can have an effect on this so either buy a couple or no players every season so the spirit remains.
The key is look at the morale and when someone gets criticised defend them, if they want a new contract offer it them and if they want to play - play them.