In the early rounds, before things get crazy, it pays to earn as many points as possible while defeating your foes. A melee attack is typically enough to dispatch a zombie, but you're better off shooting a zombie several times first. Zombies start out able to absorb four shots to the chest before a fifth one kills them. You can thus rack up points by shooting them four times, then taking them out with a melee attack. In later rounds, you can employ a similar strategy, while using more bullets before dealing the killing blow (since the zombies gradually grow tougher).