Infinite Orbs
During the Challenge of Atlas, you will come to a point after passing the saw room where you will see an enemy at the bottom of a tower. Rather than killing him, place yourself between he and the ladder going down. After killing the baddie at the top of the ladder, use your Square, Square, Triangle combo to kill the enemy near the tower. If you do this correctly, he will hit the ground and bounce back--but due to a glitch in the game he will get caught in the ladder and bounce indefinitely. As he bounces, he will continue to drop red orbs. Stand there and you will eventually build up 199 full red bars.
Quick Counters
Once you reach Blades of Chaos Level 3, you will be able to perform Hades' Revenge, a more advanced and powerful blocking maneuver. Depending on who you are fighting, you have three attacks to do maximum damage without taking unnecessary hits. In normal combat, press R1 after pressing L1. If you are heavily outnumbered, press Square to make some room between them and you, and if you are up against stronger enemies you can press Triangle to do a powerful attack against them.
Skip the Conveyors
The moment you enter the conveyor belt area before Pandora's Box, stop. There is a very small area between entering the room and triggering the fight with the Harpies. Once you're in the room, move all the way to one side (it doesn't matter which side, as they are both identical in layout). Then, guide Kratos up the screen along the conveyor belt until you reach the first pedestal. Jump on the pedestal and congratulations: you've avoided a real pain of a fight as you stroll leisurely to Pandora's Box.