To stay out of reach of enemy fire, first select a diffulty Hard on the Options screen. When the game begins, stay in the upper left or upper right corner and keep firing. This will only work in the first three levels.
Extra Lives
Use the Level Select trick to get a continue, then select Option, then hold A + B + C and press Start. If the player prompt shows a panda's face you were successful.
Level Select
At the title screen, press A once, then press Up, Down, Left, Right, Right, Left, Down, Up. Next press START for level 2. For level 3, press A twice instead of once, for level 4, press A three times.