New Game+
Unlocking the new game plus in Persona 3 allows players alternate levels and bosses a second time through the game, but starts the game with all the players items, stats and costumes from the end of the first game. To unlock new game plus, play Persona 3 through once, then save your progress after passing the last boss. Load your file. You will restart the game with all your inventory and the alternate levels will be available.
Level Up
An easy way to level up once beginning your new game plus is to return to Tarturus and find the Monad door. The villains in the Monad area are considerably stronger than other monsters in the game, allowing players to easily level up weaker characters in a short amount of time. This is especially helpful preparation to face the alternate boss at the end of the new game plus. The Monad area is only available as a free play area after you beat the game once.
Intelligence Boost
To boost your intelligence, head to the shrine in the town map before hanging out with friends after school. Go to the fortune drawing and pay the 100 yen. You will either gain or lose yen, but regardless, head to the altar and donate 1,000 yen. It will increase your intelligence and you can continue regular gameplay.
Restore Party's HP and SP
At any time in the Tartarus area, a player can warp back to the first floor. Doing so will completely restore the HP and SP of any character in your party. This method will not work for characters who are marked "tired," however, these characters can only be restored by returning to the dorm for one night.