To have infinite shurikens, go into the options menu and set the amount of shurikens to 00. Wait for a few seconds, and the 00 will change into a ∞. When you start the game, the shuriken counter will always be at 999, so you'll never run out of them.

At the beginning of the second part of round 4, there is a hidden extra life that you can reveal by shuriken spraying the bottom left hand corner of the first conveyer belt. This extra life item actually gives you two lives -so if you immediately die, you will have gained two lives whilst only losing one. Keep repeating this process to stack up a large number of lives. The counter stops at 9, but you can still keep earning them anyway. It is possible to collect the item without falling into the pit, but it takes some practice.