Easy Way To The Dark Side
When you discover the stowaway aboard the 'Ebon Hawk', select the option 'Get out of here, whoever you are! Right now!' Then, before she leaves, talk to her again and select the same option. Each time you do this you gain 1 Dark Side point each time. Repeat as desired.
100% success rate. 

Secret Hidden Ending
Have your first control pad plugged into ports 1, 2, or 3, and have another control pad plugged into port 4. Before you enter the final battle with Darth Malak (do NOT open the door), pull both the 'L' and 'R' buttons and press and hold the Y buttons on both control pads for a secret hidden ending.
0% success rate. 

Change Voice Pitch
Have your first control pad plugged into ports 1, 2, or 3, and have another control pad plugged into port 4. Press the Black or White buttons on the second control pad to change the pitch of the voices in the game.

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