Say you have a player who is on the fringes of the first team, he has the stats and quality to be in your first team, there should be a team talk thing where you can tell that player prove to me in this game that your good enough.
a good idea all round. something similar for older players, eg "go out and use your experience to help the others" would be good too.
or another good one could be where you have paid a high fee for a player, you could tell him go and prove to the world that your worth that transfer fee.
or an even better one you could use 'go out and score or i will cut you'