If you shoot an enemy when the warning reticule is on him but before he's launched his projectile you will receive a bonus.
Chickens
It is sometimes possible to shoot chickens to continue your combo to the next screen.
Restart Level
Tap 'Restart' instead of 'Respawn' if you're trying for a better score and you die. Doing this will enable you to go back to the start of the level and try again.