General:
* Warm up
* Get ready
* Prepare
* Anticipate
* Hype up
Specific to events:
* Party (for social events)
* Rehearse (for performances)
* Strategize (for competitions)
* Fuel up (for athletic events)
Informal:
* Get pumped
* Get hyped
* Get loose
* Get in the mood
The best synonym for "pregame" depends on the specific situation and the tone you want to convey.