Tip 1: If you're using the shotgun, wait until a horde of zombies is close up to do the most damage.

Tip 2: When the game snaps to a dying zombie to show you its death throes, don't stop shooting - you can still get overwhelmed in that split second.

Tip 3: If you can only afford one piece of equipment at the start of a level, make it instant reload. It's a godsend when you're surrounded.