On both offense and defense, at the playcall screen, after selecting the formation, when selecting the play, hold in both the left trigger and the right trigger AND press either (X), (A), or (B). When you do this you will feel a vibration on the controller and the play corresponding to the button pressed with both triggers held in will be stored as the audible for that button. This is a subtle but incredibly important feature, especially in multiplayer games. With the 25 second play clock, it can sometimes be hard to find the time to store a lot of plays throughout the game. But occasionally, storing a play and remembering the formation that it was under could allow the user to bust it out via pre-snap audible when the opposition is least expecting it. That could make the difference in a close game.
When I store audibles prior to the opening kickoff, I usually store the most important plays to the (L) and (R) buttons. This is because these are the only two audibles that I cannot change on the fly during the game and always try to make them important plays. (since I can only store/change audibles to the (X), (A), and (B) buttons)
Sometimes, coming out of a timeout, I'll quickly store a few plays in the same formation, then choose yet another play from that same formation. This will make it possible to audible to a stored play from within the same formation. This will make the audible more subtle since the formation won't change at the LOS.
It is also important to note that when choosing a play from the playcalling menu, the user can also bluff a play as well. (call a play that is not one of the three plays in center view) If the user holds down just the right trigger and presses (X), (A), or (B), the user actually chooses either play 1, 2, or 3 immediately below the three plays in diagram on the playcalling screen. If the user holds down just the left trigger and presses either (X), (A), or (B), the user actually chooses play 1, 2, or 3 immediately above the three plays in diagram on the playcalling screen. Just another subtle trick to keep users guessing in human vs human games.