General:
* Pre-game (most common and direct)
* Ante-game (less common, more formal)
* Prior to the game
* Beforehand
Specific to preparation:
* Warm-up (if referring to physical preparation)
* Practice
* Rehearsal (if referring to a play or performance)
* Strategizing (if focusing on planning)
More specific to a particular game:
* Pre-match (for a sporting event)
* Pre-show (for a concert or theatrical performance)
The best word will depend on the specific context of your writing.