When you fire at the base of the first conveyer belt on the second stage of level four a special Musashi symbol will appear. Get onto the conveyor belt and let it carry you to the end. You will lose a life but you will gain 2 and this can be repeated as many times as you want untill you have the desired amount of lives.
At the 'Options' screen move the pointer to Shurikins and set them to 00. If you then remain idle without moving the pointer for about 30 seconds and you will hear a sound and the '00' will change to an infinite symbol. When you now start the game you will have unlimited Shurikins.
When you have the weapons upgrade you will have a mystic shield that will protect you from small projectiles like ninja stars and bullets. The light on Shinobi's wrist is the mystic shield and is activated only when you walk. This shield will not protect you against big projectiles like the grenades the soliers throw.