be nice to them, when they play well, praise them (player interraction), don't criticise, if they want a transfer, accept, they'll soon want back when your team wins a very important match or achieves a high place in a cup or the league.
If you're afraid of losing them on free transfers, the secret is to play the guy you want to keep and whose contract will be expiring soon a bit more, praise etc, he'll soon change his status to "happy to stay at the club". That's when you offer him a new long contract. After he has accepted, you may even send him to the reserves. | 
