To choose your alternate paintjob in main game, practice or boss mode, press the R button at the plane select screen. To select the alternate color in Death Match, hold the R button while selecting one of the four original planes.
To activate Mao Mao and his F-15J Eagle, press C Left, C Down, C Right, C Up, C Left, C Rightthen C Down on the main menu. (He'll also appear if the game is continued more than three times on the Tokyo stage.)
To get Spanky and his X-29A, complete all the bonus stages.
To get a new ship press C-left, C-down, C-right, C-up, C-left, C-right, C-down at the press start screen. To get a new paint job on the planes press the Top-Right button on teh plane selection screen.
At the "PRESS START" screen, press C-Left, C-Down, C-Right, C-Up, C-Left, C-Right, C-Down.
On the title screen, (full screen logo, and "Press Start") NOT the main menu screen, press the following buttons: C-Left, C-Down, C-Right, C-Up, C-Left, C-Right, C-Down. Don't be discouraged when you don't hear a sound or see something new, because nothing will happen. Go to any plane select screen, and on the far left, you will see a new F-15 plane with a girl pilot.