If you begin running out of energy units, allow yourself to die. The moment you are brought back to life, you will get an energy unit for free. For example, if you let yourself die five times in a row you will get five energy units for free. Do this trick each time you begin a new chapter.
After you unlock all 5 of the regular endings, you will see a special prologue that will allow you to play again and unlock another ending.
After you complete the game, and one of the 5 endings run, you will then have an Extra option available at the Title Screen that allows you to view your Completion Percentage for every chapter.
Complete the game to unlock an FMV sequence.
Complete the game with 3 different endings to unlock another FMV.
Complete the game with all 5 endings and the bonus ending and unlock another FMV.
Earn 100% completion in all Chapters and to change the image on the title screen.