After playing through quite a few times I've realized a few things.

1: You move faster diagonally than left/right or up/down.
2: Zombites move slower than you but in the same fashion.
3: Only a certain number of zombies are generated depending on your kills. Once that number is reached no new zombies are spawned until you kill another one.

Using these to my advantage I realized I could herd all the current zombies into a TIGHT cluster by just moving around the screen in a diamond pattern. Then I could mow em down with the machine gun, pick up the drops and repeat. Right now I'm at 700 kills using this method, with 100 life and well over 2k ammo for my machine gun. Thought I'd share.