To get a player to play a full game, the best method I have seen is to:
1. Set that player as starter, backup, and reserve for a position.
2. Only activate the players that will be used as a starter or backup in that game, and make sure everyone is set in the lineup at the positions you want them to play.
This ensures that even in garbage time, there is no one to sub in for the player(s) you are trying to get to play the full game.
There may be other things that help, but those are the two main things I can think of at the moment.