offer them a new contract. signing that settles them down. a good run of performances/wins usually helps too. successful team 99.9% of the team equals happy players. praise can help too i had hatem ben arfa at villa, he was constantly becoming unsettled and couldnt live in the area blah blah. i praised him nearly every week for a season, next season he signed a new contract, settled down and won world player of the year!!!!