> Successfully complete the first three Missions without running out of continues to unlock Missions 4, 5 and 6.  The extra missions are set within the same areas, but are more challenging.
(courtesy: GameSages)

> Play as Bruno:
Collect all of the illustrations in the game to unlock old-school Bruno from Die Hard Arcade (ST-V, Saturn).

> Play as Cindy:
Select Ivy and hold Start at the Character Select screen to play as Cindy from Die Hard Arcade (ST-V, Saturn).

> Play as Monkey:
Successfully complete Missions 4, 5 and 6, which must first be unlocked (see Extra Missions).
(courtesy: GameSages)

> Pause the game and press L+A+B+Y.  Unpause it then the ammunition will be unlimited for every weapon.
(courtesy: DC-IGN)