First rule is: reload whenever possible.
You can only use an ammo crate once per stage, except in Survival Mode, so avoid refilling your weapons after firing just a couple of bullets.
Be on the lookout for bonus crates.
Choose the best weapons for missions.
Do not let the Bomber come close. Shoot him from a distance. Otherwise he will explode and deal a lot of damage.
Kill Piper as soon as possible or he will call siblings to his aid. Piper will fight back aggressively when cornered.
Move fast when Thrower begins to launch his projectiles. Be sure to attack him when he is trying to fall back and reload.
SH Security: Former security officers' armor makes them tough. These guys are hard to kill.
Zombie scientists cannot be stunned and will advance until they are killed.
You'll receive free gold for nabbing in-game achievements. These range from rescuing survivors to killing 110 common zombies. To view the current list, just press the Goals tab on the top left corner of the map screen.