Skip intermission sequences:
During an intermission, hold down Triangle.

Guitar mini-game:
Go to the screen where you choose your song. Press different controller buttons to display little icons on the screen that make the song do different things:

Hide icon:
Press Red, Blue, Green, Blue, Red, Green for hidden mode. When the bars get to the middle/top area, they disappear making it very hard to hit them correctly.

FF icon:
Press Red, Green, Blue, Pick(2) for fast forward mode. This makes the bars scroll faster.

SFF icon:
Press Red, Green, Blue, Pick(2), Red, Green, Blue, Pick(2). The bars will scroll even faster.

Random icon:
Press Blue, Green(2), Red, Green, Pick for random mode. Some notes will be randomized.

SUD icon:
Press Green, Red, Blue, Green, Blue, Red, Pick at the mode selection screen. Notes will appear closer to the play line.

Little icon:
Press Red(3), Green(2), Blue(3), Pick at the mode selection screen. This mode only allows two chords.