Typically it devolves into "Fuck it, I'll just add more than I'd expected to add to make sure they actually have a hard time.."
Most recently, we played on a 2x4 strip involving a long road, buildings either side and spawn-points for zombies all down the road.
The players were swamped throughout the entire game, and even suffered casualties! (A huge change!)
We even had a tank tearing it up and smashing camper-vans at the players!
Mankinds first mistake; Questioning why those around him, are dying.