Repeatedly press A + B when low on energy to keep it from going down faster.
Auto turtle selection:
Press Down(5) at the character selection screen, or Down(10) at the same screen for two player mode. After a turtle is defeated, the next turtle will automatically be selected randomly. Alternately, rapidly tap Left, Right repeatedly at the character selection screen.
Press Up(2), Down(2), Left, Right, Left, Right, A, B, Start at the title screen before the demonstration starts. Note: For two player mode press Up(2), Down(2), Left, Right, Left, Right, A, B, Select, Start. Level select, extra lives, and sound test options will now be available.