It is only possible to hold a limited amount of health restoratives, these are the 'Grains of Spiritual Life' and the 'Herb of Spiritual Life'. If you use a 'Lives Of The Thousand Gods' however you will notice that it will restore and increase your health meter.
To perform this trick you trigger an enemy and then retreat to a 'Save' spot and save your game. If you now reload the game and return to the location where you triggered the enemies they will no longer be there. Due to 'Save' spots being blocked off this cheat will not work in some locations.
In Chapter 4 enter the Statue of Liberty museum across from the Test of Valor. Slice open the wall that looks like it is made of straw to find a silver Xbox. This item has the ability to fully restore your health an unlimited amount of times.
Press the 'Up' key on the D-Pad and go to 'Weapon' select. Then pick a different weapon but press A + B instead of just A. When this code has been enabled you will be invincible but will only be able to attack with the guillotine throw.
When you have completed the game on any difficulty setting and saved it when you are prompted you will retain ALL weapons, life, ninpo, items and essence you had in that game when you begin a new game on the same difficulty setting using that saved file.