Multiple Aircraft:
On all levels that contain multiple aircraft bogies, the best way to take them out is to fly low to the ground and swoop up to take one out at a time.

Anti-aircraft Guns:
Flying close to the ground causes the anti-aircraft guns to overshoot your plane. They cannot fire below a certain point.

All Levels & Planes:
At the player name screen, enter SHPONGLE

Thanks to Revolution readers The Gooseman and Jim!