At the main menu, press Z, R, L, B, X, Y. If you entered the code correctly, a voice will say "Go cheese". Start a new game to begin with the Soul Reaver and Iron Armor.
Immediately after clearing a checkpoint, if you are low on health, reload the game. You will be back exactly where you just were, except your health meter is now completely full.
After defeating Sebastian and acquiring the Dark Gift Charm, use it on a enemy that you cannot charm. It will appear as if they have a headache. Quickly run towards the enemy and attack. Repeat this until the enemy is killed.