Have the transfer window open. For example, if playing as Manchester United, put Tevez on the transfer market and negotiate for someone like Eto'o, who is better. If you get him, something will appear if someone bid for Tevez. Let Tevez go because you got Eto'o. Instead of paying all that money for him, you are only losing a little bit.
Sign great players for higher wages and set their contract length to four years. When they are on your team, go to "Team Management", then "Player Contracts". Go to those good players and reduce their wage as much as desired. Sign them for five years and they will accept as long as the wages are not too low.