In the Cheats menu, select "Enter new cheat" and hit the frets(one at a time) in a specific order to unlock something.

Effect	Password
Air Instruments	Blue, Yellow, Blue, Red(2), Yellow, Green, Yellow.
All Hammer Ons/Pull Offs	Red, Green, Blue, Green, Blue, Green, Red, Green.
All Slider Gems	Yellow, Green, Yellow(3), Red, Blue, Red.
Auto Kick	Yellow, Green, Yellow, Blue(2), Red, Blue, Red.
Electrika Steel	Blue(2), Red, Yellow, Red, Yellow, Blue(2).
Focus Mode	Yellow(2), Green(2), Red(2), Blue(2).
HUD Free Mode	Green, Red, Green, Red, Yellow, Blue, Green, Red.
Invisible Characters	Green, Red, Yellow, Green, Yellow, Blue, Yellow, Green.
Most Characters	Blue, Yellow, Green, Yellow, Red, Green, Red, Yellow.
Performance Mode	Yellow(2), Blue, Green, Blue, Red(3).