Essentially it boils down to how well a player has played in the last couple of games. To increase your player's form you'll want to make sure you stay in position, complete your passes and don't give up possession, make your shots on goal (force a keeper save or score), win tackles (don't make unnecessary tackles, etc. At the end of the game there is a little window that pops up that will show you your player's statistics for the game (completed passes, shots/shots on goal, tackles, etc). Take a look at that screen to see what you need to improve upon.